Beamformed transmission in high efficiency wireless LAN

ABSTRACT

The present invention relates to a method and apparatus for beamformed transmission in a wireless local area network. According to one aspect of the present invention, a method for transmitting a Physical layer Protocol Data Unit (PPDU) frame to a plurality of stations (STAs) by an Access Point (AP) in a wireless local area network may be provided. The method may include transmitting a SIGNAL field of the PPDU frame, the SIGNAL field including beamforming information indicating whether beamforming is applied to respective data units of the PPDU frame, and transmitting the data units of the PPDU frame, the data units being individually beamformed or not beamformed according to the beamforming information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefits of U.S. Provisional Application No.62/060,429, filed Oct. 6, 2014, and U.S. Provisional Application No.62/159,175, filed on May 8, 2015, which are hereby incorporated byreference as if fully set forth herein.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a Wireless Local Area Network (WLAN)and, more particularly, to a method, apparatus, and software forbeamformed transmission in a high efficiency WLAN, and a recordingmedium in which the software is stored.

Discussion of the Related Art

Along with the recent development of information and telecommunicationtechnology, various wireless communication techniques have beendeveloped. Among them, the WLAN enables a user to wirelessly access theInternet based on radio frequency technology in a home, an office, or aspecific service area using a portable terminal such as a PersonalDigital Assistant (PDA), a laptop computer, a Portable Multimedia Player(PMP), a smartphone, etc.

To overcome limitations in communication speed that the WLAN faces, therecent technical standards have introduced a system that increases thespeed, reliability, and coverage of a wireless network. For example, theInstitute of Electrical and Electronics Engineers (IEEE) 802.11nstandard has introduced Multiple Input Multiple Output (MIMO) that isimplemented using multiple antennas at both a transmitter and a receiverin order to support High Throughput (HT) at a data processing rate of upto 540 Mbps, minimize transmission errors, and optimize data rates.

In recent times, to support increased numbers of devices supportingWLAN, such as smartphones, more Access Points (APs) have been deployed.Despite increase in use of WLAN devices supporting the Institute ofElectrical and Electronics Engineers (IEEE) 802.11 ac standard, thatprovide high performance relative to WLAN devices supporting the legacyIEEE 802.11g/n standard, a WLAN system supporting higher performance isrequired due to WLAN users' increased use of high volume content such asa ultra high definition video. Although a conventional WLAN system hasaimed at increase of bandwidth and improvement of a peak transmissionrate, actual users thereof could not feel drastic increase of suchperformance.

In a task group called IEEE 802.11 ax, High Efficiency WLAN (HEW)standardization is under discussion. The HEW aims at improvingperformance felt by users demanding high-capacity, high-rate serviceswhile supporting simultaneous access of numerous stations in anenvironment in which a plurality of APs is densely deployed and coverageareas of APs overlap.

However, a detailed scheme for performing beamformed transmission in theHEW has not been decided yet.

SUMMARY OF THE INVENTION

Objects of the present invention are to provide a method and apparatusfor performing beamformed transmission in a WLAN supporting downlinkmulti-user transmission or uplink multi-user transmission.

The objects of the present invention are not limited to the foregoingdescriptions, and additional objects will become apparent to thosehaving ordinary skill in the pertinent art to the present inventionbased upon the following description.

In accordance with an aspect of the present invention, a method fortransmitting a Physical layer Protocol Data Unit (PPDU) frame to aplurality of stations (STAs) by an Access Point (AP) in a wireless localarea network may be provided. The method may include transmitting aSIGNAL field of the PPDU frame, the SIGNAL field including beamforminginformation indicating whether beamforming is applied to respective dataunits of the PPDU frame; and transmitting the data units of the PPDUframe, the data units being individually beamformed or not beamformedaccording to the beamforming information.

In accordance with another aspect of the present invention, a method fortransmitting a Physical layer Protocol Data Unit (PPDU) frame to anAccess Point (AP) by a station (STA) in a wireless local area networkmay be provided. The method may include receiving a trigger frameeliciting the PPDU frame simultaneously transmitted from a plurality ofSTAs including the STA and at least one other STA, the trigger frameincluding beamforming information indicating whether beamforming isapplied to respective data units of the PPDU frame; and transmitting thePPDU frame including a data unit of the STA, the data unit of the STAbeing individually beamformed or not beamformed according to thebeamforming information.

In accordance with another aspect of the present invention, an AccessPoint (AP) for transmitting a Physical layer Protocol Data Unit (PPDU)frame to a plurality of stations (STAs) in a wireless local area networkmay be provided. The AP may include a baseband processor, a RadioFrequency (RF) transceiver, and a memory. The baseband processor may beconfigured to transmit a SIGNAL field of the PPDU frame using the RFtransceiver, the SIGNAL field including beamforming informationindicating whether beamforming is applied to respective data units ofthe PPDU frame; and to transmit the data units of the PPDU frame usingthe transceiver, the data units being individually beamformed or notbeamformed according to the beamforming information.

In accordance with another aspect of the present invention, a station(STA) for transmitting a Physical layer Protocol Data Unit (PPDU) frameto an Access Point (AP) in a wireless local area network may beprovided. The STA may include a baseband processor, a Radio Frequency(RF) processor, and a memory. The baseband processor may be configuredto receive a trigger frame eliciting the PPDU frame simultaneouslytransmitted from a plurality of STAs including the STA and at least oneother STA using the transceiver, the trigger frame including beamforminginformation indicating whether beamforming is applied to respective dataunits of the PPDU frame; and to transmit the PPDU frame including a dataunit of the STA using the transceiver, the data unit of the STA beingindividually beamformed or not beamformed according to the beamforminginformation.

In accordance with another aspect of the present invention, software ora computer-readable medium having executable instructions for an AccessPoint (AP) to transmit a Physical layer Protocol Data Unit (PPDU) frameto a plurality of stations (STAs) in a wireless local area network maybe provided. The executable instructions may cause the AP to transmit aSIGNAL field of the PPDU frame, the SIGNAL field including beamforminginformation indicating whether beamforming is applied to respective dataunits of the PPDU frame; and to transmit the data units of the PPDUframe, the data units being individually beamformed or not beamformedaccording to the beamforming information.

In accordance with another aspect of the present invention software or acomputer-readable medium having executable instructions for a station(STA) to transmit a Physical layer Protocol Data Unit (PPDU) frame to anAccess Point (AP) in a wireless local area network may be provided. Theexecutable instructions may cause the STA to receive a trigger frameeliciting the PPDU frame simultaneously transmitted from a plurality ofSTAs including the STA and at least one other STA, the trigger frameincluding beamforming information indicating whether beamforming isapplied to respective data units of the PPDU frame; and to transmit thePPDU frame including a data unit of the STA, the data unit of the STAbeing individually beamformed or not beamformed according to thebeamforming information.

It is to be understood that both the foregoing summarized features areexemplary aspects of the following detailed description of the presentinvention without limiting the scope of the present invention.

According to the present invention, a method and apparatus forperforming beamformed transmission in a WLAN supporting downlinkmulti-user transmission or uplink multi-user transmission are provided.

The advantages of the present invention are not limited to the foregoingdescriptions, and additional advantages will become apparent to thosehaving ordinary skill in the pertinent art to the present inventionbased upon the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a block diagram of a Wireless Local Area Network (WLAN)device;

FIG. 2 is a schematic block diagram of an exemplary transmission signalprocessing unit in a WLAN;

FIG. 3 is a schematic block diagram of an exemplary reception signalprocessing unit in a WLAN;

FIG. 4 depicts a relationship between InterFrame Spaces (IFSs);

FIG. 5 is a conceptual diagram illustrating a procedure for transmittinga frame in Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) to avoid collision between frames on a channel;

FIG. 6 depicts an exemplary frame structure in a WLAN system;

FIG. 7 depicts an exemplary HE PPDU frame format.

FIG. 8 depicts an exemplary High Efficiency (HE) Physical layer ProtocolData Unit (PPDU) frame format according to the present invention;

FIG. 9 depicts subchannel allocation in a HE PPDU frame format accordingto the present invention;

FIG. 10 depicts a subchannel allocation method according to the presentinvention;

FIG. 11 depicts the starting and ending points of an High EfficiencyLong Training Field (HE-LTF) field in a HE PPDU frame format accordingto the present invention;

FIG. 12 depicts a High Efficiency SIGnal B (HE-SIG-B) field and a HighEfficiency SIGnal C (HE-SIG-C) field in the HE PPDU frame formataccording to the present invention;

FIG. 13 depicts another example of a HE PPDU frame format according tothe present invention;

FIG. 14 depicts an exemplary HE PPDU frame format for a wide channelband according to the present invention;

FIG. 15 depicts another exemplary HE PPDU frame format according to thepresent invention;

FIGS. 16 and 17 depict exemplary HE PPDU frame formats supportingbeamforming according to the present invention;

FIG. 18 depicts an exemplary sounding protocol according to the presentinvention;

FIG. 19 depicts an exemplary NDPA frame format according to the presentinvention;

FIGS. 20 to 22 depict additional exemplary sounding protocols accordingto the present invention;

FIG. 23 depicts another exemplary sounding protocol according to thepresent invention;

FIGS. 24 and 25 depict exemplary DL HE PPDU frame formats supporting DLbeamforming transmission according to the present invention;

FIGS. 26 to 30 depict exemplary trigger frame formats and UL HE PPDUframe formats supporting UL beamforming according to the presentinvention;

FIG. 31 is a flowchart depicting an exemplary method according to thepresent invention; and

FIG. 32 is a flowchart depicting another exemplary method according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, only certain embodiments of thepresent invention have been shown and described, simply by way ofillustration. As those skilled in the art would realize, the describedembodiments may be modified in various different ways, all withoutdeparting from the spirit or scope of the present invention.Accordingly, the drawings and description are to be regarded asillustrative in nature and not restrictive. Like reference numeralsdesignate like elements throughout the specification.

In a Wireless Local Area network (WLAN), a Basic Service Set (BSS)includes a plurality of WLAN devices. A WLAN device may include a MediumAccess Control (MAC) layer and a PHYsical (PHY) layer in conformance toInstitute of Electrical and Electronics Engineers (IEEE) 802.11 seriesstandards. At least one of the WLAN devices may be an Access Point (AP)and the other WLAN devices may be non-AP Stations (non-AP STAs).Alternatively, all of the WLAN devices may be non-AP STAs in an ad-hocnetwork. Generally, the term STA covers AP STA and non-AP STA. However,only a non-AP STA may be referred to as a STA, for the convenience'ssake.

FIG. 1 is a block diagram of a WLAN device.

Referring to FIG. 1, a WLAN device 1 includes a baseband processor 10, aRadio Frequency (RF) transceiver 20, an antenna unit 30, a memory 40, aninput interface unit 50, an output interface unit 60, and a bus 70.

The baseband processor 10 may be simply referred to as a processor,performs baseband signal processing described in the presentspecification, and includes a MAC processor (or MAC entity) 11 and a PHYprocessor (or PHY entity) 15.

In an embodiment of the present invention, the MAC processor 11 mayinclude a MAC software processing unit 12 and a MAC hardware processingunit 13. The memory 40 may store software (hereinafter referred to as‘MAC software’) including at least some functions of the MAC layer. TheMAC software processing unit 12 may execute the MAC software toimplement some functions of the MAC layer, and the MAC hardwareprocessing unit 13 may implement the remaining functions of the MAClayer as hardware (hereinafter referred to as ‘MAC hardware’). However,the MAC processor 11 is not limited to the foregoing implementationexamples.

The PHY processor 15 includes a transmission signal processing unit 100and a reception signal processing unit 200.

The baseband processor 10, the memory 40, the input interface unit 50,and the output interface unit 60 may communicate with one another viathe bus 70.

The RF transceiver 20 includes an RF transmitter 21 and an RF receiver22.

The memory 40 may further store an Operating System (OS) andapplications. The input interface unit 50 receives information from auser, and the output interface unit 60 outputs information to the user.

The antenna unit 30 includes one or more antennas. When Multiple inputMultiple Output (MIMO) or Multi-User MIMO (MU-MIMO) is used, the antennaunit 30 may include a plurality of antennas.

FIG. 2 is a schematic block diagram of an exemplary transmission signalprocessor in a WLAN.

Referring to FIG. 2, the transmission signal processing unit 100includes an encoder 110, an interleaver 120, a mapper 130, an InverseFourier Transform (IFT) processor 140, and a Guard Interval (GI)inserter 150.

The encoder 110 encodes input data. For example, the encoder 100 may bea Forward Error Correction (FEC) encoder. The FEC encoder may include aBinary Convolutional Code (BCC) encoder followed by a puncturing device,or the FEC encoder may include a Low-Density Parity-Check (LDPC)encoder.

The transmission signal processing unit 100 may further include ascrambler for scrambling input data before encoding to reduce theprobability of long sequences of 0s or 1s. If a BCC encoding scheme isused in the encoder 110, the transmission signal processing unit 100 mayfurther include an encoder parser for demultiplexing the scrambled bitsamong a plurality of BCC encoders. If an LDPC encoding scheme is used inthe encoder 110, the transmission signal processing unit 100 may not usethe encoder parser.

The interleaver 120 interleaves the bits of each stream output from theencoder 110 to change orders of bits. Interleaving may be applied onlywhen a BCC encoding scheme is used in the encoder 110. The mapper 130maps a sequence of bits output from the interleaver 120 to constellationpoints. If an LDPC encoding scheme is used in the encoder 110, themapper 130 may further perform LDPC tone mapping besides theconstellation point mapping.

In MIMO or MU-MIMO, the transmission signal processing unit 100 may useas many interleavers 120 as and as many mappers 130 as the number N_(SS)of spatial streams. In this case, the transmission signal processingunit 100 may further include a stream parser for dividing the outputs ofthe BCC encoders or the output of the LDPC encoder into a plurality ofblocks to be provided to the different interleavers 120 or mappers 130.The transmission signal processing unit 100 may further include aSpace-Time Block Code (STBC) encoder for spreading the constellationpoints from N_(SS) spatial streams into N_(STS) space-time streams and aspatial mapper for mapping the space-time streams to transmit chains.The spatial mapper may use direct mapping, spatial expansion, orbeamforming.

The IFT processor 140 converts a block of constellation points outputfrom the mapper 130 or the spatial mapper to a time-domain block (i.e.,a symbol) by Inverse Discrete Fourier Transform (IDFT) or Inverse FastFourier Transform (IFFT). If the STBC encoder and the spatial mapper areused, the IFT processor 140 may be provided for each transmit chain.

In MIMO or MU-MIMO, the transmission signal processing unit 100 mayinsert Cyclic Shift Diversities (CSDs) in order to prevent unintendedbeamforming. A CSD insertion may applied before or after IFT. A CSD maybe specified for each transmit chain or for each space-time stream.Alternatively, the CSD may be applied as a part of the spatial mapper.

In MU-MIMO, some blocks before the spatial mapper may be provided foreach user.

The GI inserter 150 prepends a GI to a symbol. The transmission signalprocessing unit 100 may optionally perform windowing to smooth edges ofeach symbol after inserting the GI. The RF transmitter 21 converts thesymbols into an RF signal and transmits the RF signal via the antennaunit 30. In MIMO or MU-MIMO, the GI inserter 150 and the RF transmitter21 may be provided for each transmit chain.

FIG. 3 is a schematic block diagram of an exemplary a reception signalprocessor in a WLAN.

Referring to FIG. 3, the reception signal processing unit 200 includes aGI remover 220, a Fourier Transform (FT) processor 230, a demapper 240,a deinterleaver 250, and a decoder 260.

The RF receiver 22 receives an RF signal via the antenna unit 30 andconverts the RF signal into symbols. The GI remover 220 removes a GIfrom the symbols. In MIMO or MU-MIMO, the RF receiver 22 and the GIremover 220 may be provided for each receive chain.

The FT 230 converts the symbol (i.e., the time-domain block) into ablock of constellation points by Discrete Fourier Transform (DFT) orFast Fourier Transform (FFT). The FT processor 230 may be provided foreach receive chain.

In MIMO or MU-MIMO, the reception signal processing unit 200 may includea spatial demapper for converting Fourier Transformed receiver chains toconstellation points of space-time streams, and an STBC decoder fordespreading the constellation points from the space-time streams intothe spatial streams.

The demapper 240 demaps constellation points output from the FTprocessor 230 or the STBC decoder to bit streams. If an LDPC encodingscheme has been applied to the received signal, the demapper 240 mayfurther perform LDPC tone demapping before the constellation demapping.The deinterleaver 250 deinterleaves the bits of each of the streamsoutput from the demapper 240. Deinterleaving may be applied only when aBCC encoding scheme has been applied to the received signal.

In MIMO or MU-MIMO, the reception signal processing unit 200 may use asmany demappers 240 as and as many deinterleavers 250 as the number ofspatial streams. In this case, the reception signal processing unit 200may further include a stream deparser for combining streams output fromthe deinterleavers 250.

The decoder 260 decodes streams output from the deinterleaver 250 or thestream deparser. For example, the decoder 100 may be an FEC decoder. TheFEC decoder may include a BCC decoder or an LDPC decoder. The receptionsignal processing unit 200 may further include a descrambler fordescrambling the decoded data. If a BCC decoding scheme is used in thedecoder 260, the reception signal processing unit 200 may furtherinclude an encoder deparser for multiplexing data decoded by a pluralityof BCC decoders. If an LDPC decoding scheme is used in the decoder 260,the reception signal processing unit 200 may not use the encoderdeparser.

In a WLAN system, Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) is a basic MAC access mechanism. The CSMA/CA mechanism isreferred to as Distributed Coordination Function (DCF) of IEEE 802.11MAC, shortly as a ‘listen before talk’ access mechanism. According tothe CSMA/CA mechanism, an AP and/or a STA may sense a medium or achannel for a predetermined time before starting transmission, that is,may perform Clear Channel Assessment (CCA). If the AP or the STAdetermines that the medium or channel is idle, it may start to transmita frame on the medium or channel. On the other hand, if the AP and/orthe STA determines that the medium or channel is occupied or busy, itmay set a delay period (e.g., a random backoff period), wait for thedelay period without starting transmission, and then attempt to transmita frame. By applying a random backoff period, a plurality of STAs areexpected to attempt frame transmission after waiting for different timeperiods, resulting in minimizing collisions.

FIG. 4 depicts a relationship between InterFrame Spaces (IFSs).

WLAN devices may exchange data frames, control frames, and managementframes with each other.

A data frame is used for transmission of data to be forwarded to ahigher layer. After a Distributed Coordination Function IFS (DIFS) froma time when a medium gets idle, a WLAN device performs a backoff andthen transmits a data frame. A management frame is used for exchangingmanagement information which is not forwarded to the higher layer. Afteran IFS such as the DIFS or a Point Coordination Function IFS (PIFS), theWLAN device transmits the management frame. Subtype frames of themanagement frame include a beacon frame, an association request/responseframe, a probe request/response frame, and an authenticationrequest/response frame. A control frame is used for controlling accessto the medium. Subtype frames of the control frame include aRequest-To-Send (RTS) frame, a Clear-To-Send (CTS) frame, and anACKnowledgement (ACK) frame. If the control frame is not a responseframe to another frame, the WLAN device performs a backoff after theDIFS and then transmits the control frame; or if the control frame is aresponse frame to another frame, the WLAN device transmits the controlframe after a Short IFS (SIFS) without a backoff. The type and subtypeof a frame may be identified by a type field and a subtype field in aFrame Control (FC) field.

On the other hand, a Quality of Service (QoS) STA may perform a backoffafter an Arbitration IFS (AIFS) for Access Category (AC), i.e., AIFS[i](i is determined based on AC) and then transmit a frame. In this case,the AIFC[i] may be used for a data frame, a management frame, or acontrol frame that is not a response frame.

In the example illustrated in FIG. 4, upon generation of a frame to betransmitted, a STA may transmit the frame immediately, if it determinesthat the medium is idle for the DIFS or AIFS[i] or longer. The medium isbusy for a time period during which the STA transmits the frame. Duringthe time period, upon generation of a frame to be transmitted, anotherSTA may defer access by confirming that the medium is busy. If themedium gets idle, the STA that intends to transmit the frame may performa backoff operation after a predetermined IFS in order to minimizecollision with any other STA. Specifically, the STA that intends totransmit the frame selects a random backoff count, waits for a slot timecorresponding to the selected random backoff count, and then attempttransmission. The random backoff count is determined based on aContention Window (CW) parameter and the medium is monitoredcontinuously during count-down of backoff slots (i.e. decrement abackoff count-down) according to the determined backoff count. If theSTA monitors the medium as busy, the STA discontinues the count-down andwaits, and then, if the medium gets idle, the STA resumes thecount-down. If the backoff slot count reaches 0, the STA may transmitthe next frame.

FIG. 5 is a conceptual diagram illustrating a CSMA/CA-based frametransmission procedure to avoid collision between frames on a channel.

Referring FIG. 5, a first STA (STA1) is a transmitting WLAN devicehaving data to be transmitted, a second STA (STA2) is a receiving WLANdevice to receive the data from STA1, and a third STA (STA3) is a WLANdevice located in an area where STA3 may receive a frame from STA1and/or STA2.

STA1 may determine whether a channel is busy by carrier sensing. STA1may determine channel occupancy based on an energy level of the channelor a correlation between signals on the channel, or using a NetworkAllocation Vector (NAV) timer.

If STA1 determines that the channel is not used by other devices duringa DIFS (that is, the channel is idle), STA1 may transmit an RTS frame toSTA2 after performing a backoff. Upon receipt of the RTS frame, STA2 maytransmit a CTS frame as a response to the CTS frame after a SIFS.

Upon receipt of the RTS frame, STA3 may set a NAV timer for atransmission duration of following frames (e.g., a SIFS time+a CTS frameduration+a SIFS time+a data frame duration+a SIFS time+an ACK frameduration), based on duration information included in the RTS frame. Uponreceipt of the CTS frame, STA3 may set the NAV timer for a transmissionduration of following frames (e.g., a SIFS time+a data frame duration+aSIFS time+an ACK frame duration), based on duration information includedin the CTS frame. Upon receipt of a new frame before the NAV timerexpires, STA3 may update the NAV timer based on duration informationincluded in the new frame. STA3 does not attempt to access the channeluntil the NAV timer expires.

Upon receipt of the CTS frame from STA2, STA1 may transmit a data frameto STA2 a SIFS after the CTS frame has been completely received. Uponsuccessful receipt of the data frame from STA1, STA2 may transmit an ACKframe as a response to the data frame after a SIFS.

Upon expiration of the NAV timer, STA3 may determine whether the channelis busy by carrier sensing. If STA3 determines that the channel is notin use by the other devices during a DIFS after expiration of the NAVtimer, STA3 may attempt channel access after a convention windowaccording a random backoff-based CW.

FIG. 6 depicts an exemplary frame structure in a WLAN system.

PHY layer may prepare a transmission MAC PDU (MPDU) in response to aninstruction (or a primitive, which is a set of instructions or a set ofparameters) by the MAC layer. For example, upon receipt of aninstruction requesting transmission start from the MAC layer, the PHYlayer may switch to a transmission mode, construct a frame withinformation (e.g., data) received from the MAC layer, and transmit theframe.

Upon detection of a valid preamble in a received frame, the PHY layermonitors a header of the preamble and transmits an instructionindicating reception start of the PHY layer to the MAC layer.

Information is transmitted and received in frames in the WLAN system.For this purpose, a Physical layer Protocol Data Unit (PPDU) frameformat is defined.

A PPDU frame may include a Short Training Field (STF) field, a LongTraining Field (LTF) field, a SIGNAL (SIG) field, and a Data field. Themost basic (e.g., a non-High Throughput (non-HT)) PPDU frame may includeonly a Legacy-STF (L-STF) field, a Legacy-LTF (L-LTF) field, a SIGfield, and a Data field. Additional (or other types of) STF, LTF, andSIG fields may be included between the SIG field and the Data fieldaccording to the type of a PPDU frame format (e.g., an HT-mixed formatPPDU, an HT-greenfield format PPDU, a Very High Throughput (VHT) PPDU,etc.).

The STF is used for signal detection, Automatic Gain Control (AGC),diversity selection, fine time synchronization, etc. The LTF field isused for channel estimation, frequency error estimation, etc. The STFand the LTF fields may be referred to as signals for OFDM PHY layersynchronization and channel estimation.

The SIG field may include a RATE field and a LENGTH field. The RATEfield may include information about a modulation scheme and coding rateof data. The LENGTH field may include information about the length ofthe data. The SIG field may further include parity bits, SIG TAIL bits,etc.

The Data field may include a SERVICE field, a Physical layer ServiceData Unit (PSDU), and PPDU TAIL bits. When needed, the Data field mayfurther include padding bits. A part of the bits of the SERVICE fieldmay be used for synchronization at a descrambler of a receiver. The PSDUcorresponds to a MAC PDU defined at the MAC layer and may include datagenerated/used in a higher layer. The PPDU TAIL bits may be used toreturn an encoder to a zero state. The padding bits may be used to matchthe length of the Data filed in predetermined units.

A MAC PDU is defined according to various MAC frame formats. A basic MACframe includes a MAC header, a frame body, and a Frame Check Sequence(FCS). The MAC frame includes a MAC PDU and may be transmitted andreceived in the PSDU of the data part in the PPDU frame format.

The MAC header includes a Frame Control field, a Duration/Identifier(ID) field, an Address field, etc. The Frame Control field may includecontrol information required for frame transmission/reception. TheDuration/ID field may be set to a time for transmitting the frame. Fordetails of Sequence Control, QoS Control, and HT Control subfields ofthe MAC header, refer to the IEEE 802.11-2012 technical specification.

The Frame Control field of the MAC header may include Protocol Version,Type, Subtype, To DS, From DS, More Fragment, Retry, Power Management,More Data, Protected Frame, and Order subfields. For the contents ofeach subfield in the Frame Control field, refer to the IEEE 802.11-2012technical specification.

A Null-Data Packet (NDP) frame format is a frame format that does notinclude a data packet. In other words, the NDP frame format includesonly a Physical Layer Convergence Protocol (PLCP) header part (i.e., theSTF, LTF, and SIG fields) of the general PPDU frame format, without theremaining part (i.e., the Data field) of the general PPDU frame format.The NDP frame format may be referred to as a short frame format.

The IEEE 802.11 ax task group is discussing a WLAN system, called a HighEfficiency WLAN (HEW) system, that operates in 2.4 GHz or 5 GHz andsupports a channel bandwidth (or channel width) of 20 MHz, 40 MHz, 80MHz, or 160 MHz. The present invention defines a new PPDU frame formatfor the IEEE 802.11ax HEW system. The new PPDU frame format may supportMU-MIMO or OFDMA. A PPDU of the new format may be referred to as a ‘HEWPPDU’ or ‘HE PPDU’ (similarly, HEW xyz may be referred to as ‘HE xyz’ or‘HE-xyz’ in the following descriptions).

In present specification, the term ‘MU-MIMO or OFDMA mode’ includesMU-MIMO without using OFDMA, or OFDMA mode without using MU-MIMO in anorthogonal frequency resource, or OFDMA mode using MU-MIMO in anorthogonal frequency resource.

FIG. 7 depicts an exemplary HE PPDU frame format.

A transmitting STA may generate a PPDU frame according to the HE PPDUframe format as illustrated in FIG. 7 and transmit the PPDU frame to areceiving STA. The receiving STA may detect a PPDU and then process thePPDU.

The HE PPDU frame format may broadly include two parts: the first partincluding an L-STF field, an L-LTF field, an L-SIG field, an RL-SIGfield, a HE-SIG-A field, and a HE-SIG-B field and the second partincluding a HE-STF field, a HE-LTF field, and a HE-DATA field. 64-FFTbased on a channel bandwidth of 20 MHz may be applied to the first partand a basic subcarrier spacing of 312.5 kHz and a basic DFT period of3.2 μs may be included in the first part. 256-FFT based on a channelbandwidth of 20 MHz may be applied to the second part and a basicsubcarrier spacing of 75.125 kHz and a basic DFT period of 12.8 μs maybe included in the second part.

The HE-SIG-A field may include N_(HESIGA) symbols, the HE-SIG-B fieldmay include N_(HESIGB) symbols, the HE-LTF field may include N_(HELTF)symbols, and the HE-DATA field may include N_(DATA) symbols.

A detailed description of the fields included in the HE PPDU frameformat is given in Table 1.

TABLE 1 DFT Subcarrier Element definition duration period GI spacingDescription Legacy(L)- Non-high 8 μs — — equivalent to L-STF of anon-trigger-based STF throughput(HT) 1,250 kHz PPDU has a periodicity of0.8 μs Short Training with 10 periods. field L-LTF Non-HT Long 8 μs 3.2μs 1.6 μs 312.5 kHz Training field L-SIG Non-HT 4 μs 3.2 μs 0.8 μs 312.5kHz SIGNAL field RL-SIG Repeated Non- 4 μs 3.2 μs 0.8 μs 312.5 kHz HTSIGNAL field HE-SIG-A HE SIGNAL A N_(HESIGA) * 3.5 μs 0.8 μs 312.5 kHzHE-SIG-A is duplicated on each field 4 μs 20 MHz segment after thelegacy preamble to indicate common control information. N_(HESIGA) meansthe number of OFDM symbols of the HE-SIG-A field and is equal to 2 or 4.HE-SIG-B HE SIGNAL B N_(HESIGB) * 3.2 μs 0.8 μs 312.5 kHz N_(HESIGB)means the number of field 4 μs OFDM symbols of the HE-SIG-B field and isvariable. DL MU packet contains HE-SIG- B. SU packets and UL Triggerbased packets do not contain HE-SIG-B. HE-STF HE Short 4 or 8 μs — —non- HE-STF of a non-trigger-based Training field trigger- PPDU has aperiodicity of 0.8 μs based with 5 periods. A non-trigger-based PPDU:PPDU is not sent in response to a (equivalent trigger frame. to) 1,250kHz; The HE-STF of a trigger-based trigger- PPDU has a periodicity of1.6 μs based with 5 periods. A trigger-based PPDU: PPDU is an UL PPDUsent in (equivalent response to a trigger frame. to) 625 kHz HE-LTF HELong N_(HELTF) * 2xLTF: supports 2xLTF: HE PPDU shall support 2xLTFTraining field (DTF 6.4 μs 0.8, 1.6, (equivalent mode and 4xLTF mode.period + 4xLTF: 3.2 μs to) 156.25 kHz; In the 2xLTF mode, HE-LTF GI) μs12.8 μs 4xLTF: symbol excluding GI is equivalent 78.125 kHz tomodulating every other tone in an OFDM symbol of 12.8 μs excluding GI,and then removing the second half of the OFDM symbol in time domain.N_(HELTF) means the number of HE- LTF symbols and is equal to 1, 2, 4,6, 8. HE-DATA HE DATA N_(DATA) * 12.8 μs  supports 78.125 kHz N_(DATA)means the number of HE field (DTF 0.8, 1.6, data symbols. period + 3.2μs GI) μs

L-STF is a non-HT Short Training field and may have a duration of 8 μsand a subcarrier spacing equivalent to 1250 kHz. L-STF of a PPDU whichis not based on a trigger may have a periodicity of 0.8 μs with 10periods. Herein, the trigger corresponds to scheduling information forUL transmission.

L-LTF is a non-HT Long Training field and may have a duration of 8 μs, aDFT period of 3.2 μs, a Guard Interval (GI) of 1.6 μs, and a subcarrierspacing of 312.5 kHz.

L-SIG is a non-HT SIGNAL field and may have a duration of 4 μs, a DFTperiod of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of 312.5 kHz.

RL-SIG is a Repeated Non-HT SIGNAL field and may have a duration of 4μs, a DFT period of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of312.5 kHz.

L-STF, L-LTF, L-SIG, and RL-SIG may be called legacy preambles.

HE-SIG-A is a HE SIGNAL A field and may have a duration of N_(HESIGA)*4μs, a DFT period of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of312.5 kHz. HE-SIG-A may be duplicated on each 20 MHz segment after thelegacy preambles to indicate common control information. N_(HESIGA)represents the number of OFDM symbols of the HE-SIG-A field and may havea value of 2 or 4.

HE-SIG-B is a HE SIGNAL B field and may have a duration of N_(HESIGB)*4μs, a DFT period of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of312.5 kHz. N_(HESIGB) represents the number of OFDM symbols of theHE-SIG-B field and may have a variable value. In addition, although a DLMulti-User (MU) packet may include the HE-SIG-B field, a Single-User(SU) packet and a UL trigger based packet may not include the HE-SIG-Bfield.

HE-STF is a HE Short Training field and may have a duration of 4 or 8 μsA non-trigger based PPDU may have a subcarrier spacing equivalent to1250 kHz and a trigger based PPDU may have a subcarrier spacingequivalent to 625 kHz. HE-STF of the non-triggered PPDU may have aperiodicity of 0.8 μs with 4 periods. The non-triggered PPDU is nottransmitted in response to a trigger field. HE-STF of the trigger basedPPDU may have a periodicity of 1.6 μs with 5 periods. The trigger basedPPDU is a UL PPDU transmitted in response to the trigger frame.

HE-LTF is a HE Long Training field and may have a duration ofN_(HELTF)*(DFT period+GI)μs. N_(HELTF) represents the number of HE-LTFsymbols and may have a value of 1, 2, 4, 6, or 8. A HE PPDU may supporta 2×LTF mode and a 4×LTF mode. In the 2×LTF mode, a HE-LTF symbol exceptfor a GI is equivalent to a symbol obtained by modulating every othertone in an OFDM symbol of 12.8 μs excluding a GI and then eliminatingthe first half or the second half of the OFDM symbol in the time domain.In the 4×LTF mode, a HE-LTF symbol excluding a GI are equivalent to asymbol obtained by modulating every fourth tone in an OFDM symbol of12.8 μs excluding a GI and then eliminating the first three-fourths orthe last three-fourths of the OFDM symbol in the time domain. 2×LTF mayhave a DFT period of 6.4 μs and 4×LTF may have a DFT period of 12.8 μs.A GI of HE-LTF may support 0.8 μs, 1.6 μs, and 3.2 μs 2×LTF may have asubcarrier spacing equivalent to 156.25 kHz and 4×LTF may have asubcarrier spacing of 78.125 kHz.

HE-DATA is a HE DATA field and may have a duration of, N_(DATA)*(DFTperiod+GI)μs. N_(DATA) represents the number of HE-DATA symbols. HE-DATAmay have a DFT period of 12.8 μs. A GI of HE-DATA may support 0.8 μs,1.6 μs, and 3.2 μs. HE-DATA may have a subcarrier spacing of 78.125 kHz.

The above description of the fields included in the HE PPDU frame formatmay be combined with exemplary HE PPDU frame formats described below.For example, characteristics of fields exemplarily described below maybe applied while a transmission order of the fields of the HE PPDU frameformat of FIG. 7 is maintained.

FIG. 8 depicts an exemplary HE PPDU frame format according to thepresent invention.

Referring to FIG. 8, the vertical axis represents frequency and thehorizontal axis represents time. It is assumed that frequency and timeincrease in the upward direction and the right direction, respectively.

In the example of FIG. 8, one channel includes four subchannels. AnL-STF, an L-LTF, an L-SIG, and an HE-SIG-A may be transmitted perchannel (e.g., 20 MHz), a HE-STF and a HE-LTF may be transmitted on eachsubchannel being a basic subchannel unit (e.g., 5 MHz), and a HE-SIG-Band a PSDU may be transmitted on each of subchannels allocated to a STA.A subchannel allocated to a STA may have a size required for PSDUtransmission to the STA. The size of the subchannel allocated to the STAmay be N (N=1, 2, 3, . . . ) times as large as the size of basicsubchannel unit (i.e., a subchannel having a minimum size). In theexample of FIG. 8, the size of a subchannel allocated to each STA isequal to the size of the basic subchannel unit. For example, a firstsubchannel may be allocated for PSDU transmission from an AP to STA1 andSTA2, a second subchannel may be allocated for PSDU transmission fromthe AP to STA3 and STA4, a third subchannel may be allocated for PSDUtransmission from the AP to STA5, and a fourth subchannel may beallocated for PSDU transmission from the AP to STA6.

While the term subchannel is used in the present disclosure, the termsubchannel may be referred to as Resource Unit (RU) or subband. Inparticular, the terms like OFDMA subchannel, OFDMA RU, OFDMA subband canbe used in embodiments for OFDMA in the present disclosure. Terms like abandwidth of a subchannel, a number of tones (or subcarriers) allocatedto a subchannel, a number of data tones (or data subcarriers) allocatedto a subchannel can be used to express a size of a subchannel. Asubchannel refers to a frequency band allocated to a STA and a basicsubchannel unit refers to a basic unit used to represent the size of asubchannel. While the size of the basic subchannel unit is 5 MHz in theabove example, this is purely exemplary. Thus, the basic subchannel unitmay have a size of 2.5 MHz.

In FIG. 8, a plurality of HE-LTF elements are distinguished in the timeand frequency domains. One HE-LTF element may correspond to one OFDMsymbol in time domain and one subchannel unit (i.e., a subchannelbandwidth allocated to a STA) in frequency domain. The HE-LTF elementsare logical units, and the PHY layer does not necessarily operate inunits of an HE-LTF element. In the following description, a HE-LTFelement may be referred to shortly as a HE-LTF.

A HE-LTF symbol may correspond to a set of HE-LTF elements in one OFDMsymbol in time domain and in one channel unit (e.g., 20 MHz) infrequency domain.

A HE-LTF section may correspond to a set of HE-LTF elements in one ormore OFDM symbols in time domain and in one subchannel unit (i.e., asubchannel bandwidth allocated to a STA) in frequency domain.

A HE-LTF field may be a set of HE-LTF elements, HE-LTF symbols, orHE-LTF sections for a plurality of stations.

The L-STF field is used for frequency offset estimation and phase offsetestimation, for preamble decoding at a legacy STA (i.e., a STA operatingin a system such as IEEE 802.11a/b/g/n/ac). The L-LTF field is used forchannel estimation, for the preamble decoding at the legacy STA. TheL-SIG field is used for the preamble decoding at the legacy STA andprovides a protection function for PPDU transmission of a third-partySTA (e.g., setting a NAV based on the value of a LENGTH field includedin the L-SIG field).

HE-SIG-A (or HEW SIG-A) represents High Efficiency Signal A (or HighEfficiency WLAN Signal A), and includes HE PPDU (or HEW PPDU) modulationparameters, etc. for HE preamble (or HEW preamble) decoding at a HE STA(or HEW STA). The parameters set included in the HEW SIG-A field mayinclude one or more of Very High Throughput (VHT) PPDU modulationparameters transmitted by IEEE 802.11 ac stations, as listed in [Table2] below, to ensure backward compatibility with legacy STAs (e.g., IEEE802.11ac stations).

TABLE 2 Two parts of Number VHT-SIG-A Bit Field of bits DescriptionVHT-SIG-A1 B0-B1 BW 2 Set to 0 for 20 MHz, 1 for 40 MHz, 2 for 80 MHz,and 3 for 160 MHz and 80 + 80 MHz B2 Reserved 1 Reserved. Set to 1. B3STBC 1 For a VHT SU PPDU: Set to 1 if space time block coding is usedand set to 0 otherwise. For a VHT MU PPDU: Set to 0. B4-B9 Group ID 6Set to the value of the TXVECTOR parameter GROUP_ID. A value of 0 or 63indicates a VHT SU PPDU; otherwise, indicates a VHT MU PPDU. B10-B21NSTS/Partial 12 For a VHT MU PPDU: NSTS is divided into 4 user AIDpositions of 3 bits each. User position p, where 0 ≤ p ≤ 3, uses bitsB(10 + 3p) to B(12 + 3p). The number of space- time streams for user uare indicated at user position p = USER_POSITION[u] where u = 0, 1, . .. , NUM_USERS − 1 and the notation A[b] denotes the value of array A atindex b. Zero space-time streams are indicated at positions not listedin the USER_POSITION array. Each user position is set as follows: Set to0 for 0 space-time streams Set to 1 for 1 space-time stream Set to 2 for2 space-time streams Set to 3 for 3 space-time streams Set to 4 for 4space-time streams Values 5-7 are reserved For a VHT SU PPDU: B10-B12Set to 0 for 1 space-time stream Set to 1 for 2 space-time streams Setto 2 for 3 space-time streams Set to 3 for 4 space-time streams Set to 4for 5 space-time streams Set to 5 for 6 space-time streams Set to 6 for7 space-time streams Set to 7 for 8 space-time streams B13-B21 PartialAID: Set to the value of the TXVECTOR parameter PARTIAL_AID. Partial AIDprovides an abbreviated indication of the intended recipient(s) of thePSDU (see 9.17a). B22 TXOP_PS_(—) 1 Set to 0 by VHT AP if it allowsnon-AP VHT STAs in NOT_ALLOWED TXOP power save mode to enter Doze stateduring a TXOP. Set to 1 otherwise. The bit is reserved and set to 1 inVHT PPDUs transmitted by a non-AP VHT STA. B23 Reserved 1 Set to 1VHT-SIG-A2 B0 Short GI 1 Set to 0 if short guard interval is not used inthe Data field. Set to 1 if short guard interval is used in the Datafield. B1 Short GI 1 Set to 1 if short guard interval is used andN_(SYM) mod 10 = 9; N_(SYM) otherwise, set to 0. N_(SYM) is defined in22.4.3. Disambiguation B2 SU/MU[0] 1 For a VHT SU PPDU, B2 is set to 0for BCC, 1 for LDPC Coding For a VHT MU PPDU, if the MU[0] NSTS field isnonzero, then B2 indicates the coding used for user u withUSER_POSITION[u] = 0; set to 0 for BCC and 1 for LDPC. If the MU[0] NSTSfield is 0, then this field is reserved and set to 1. B3 LDPC Extra 1Set to 1 if the LDPC PPDU encoding process (if an SU OFDM PPDU), or atleast one LDPC user's PPDU encoding process Symbol (if a VHT MU PPDU),results in an extra OFDM symbol (or symbols) as described in 22.3.10.5.4and 22.3.10.5.5. Set to 0 otherwise. B4-B7 SU VHT-MCS/MU[1- 4 For a VHTSU PPDU: 3] Coding VHT-MCS index For a VHT MU PPDU: If the MU[1] NSTSfield is nonzero, then B4 indicates coding for user u withUSER_POSITION[u] = 1: set to 0 for BCC, 1 for LDPC. If the MU[1] NSTSfield is 0, then B4 is reserved and set to 1. If the MU[2] NSTS field isnonzero, then B5 indicates coding for user u with USER_POSITION[u] = 2:set to 0 for BCC, 1 for LDPC. If the MU[2] NSTS field is 0, then B5 isreserved and set to 1. If the MU[3] NSTS field is nonzero, then B6indicates coding for user u with USER_POSITlON[u] = 3: set to 0 for BCC,1 for LDPC. If the MU[3] NSTS field is 0, then B6 is reserved and setto 1. B7 is reserved and set to 1 B8 Beamformed 1 For a VHT SU PPDU: Setto 1 if a Beamforming steering matrix is applied to the waveform in anSU transmission as described in 20.3.11.11.2, set to 0 otherwise. For aVHT MU PPDU: Reserved and set to 1 NOTE-If equal to 1 smoothing is notrecommended. B9 Reserved 1 Reserved and set to 1 B10-B17 CRC 8 CRCcalculated as in 20.3.9.4.4 with c7 in B10. Bits 0-23 of HT-SIG1 andbits 0-9 of HT-SIG2 are replaced by bits 0-23 of VHT-SIG-A1 and bits 0-9of VHT-SIG-A2, respectively. B18-B23 Tail 6 Used to terminate thetrellis of the convolutional decoder. Set to 0.

[Table 2] illustrates fields, bit positions, numbers of bits, anddescriptions included in each of two parts, VHT-SIG-A1 and VHT-SIG-A2,of the VHT-SIG-A field defined by the IEEE 802.11ac standard. Forexample, a BW (BandWidth) field occupies two Least Significant Bits(LSBs), B0 and B1 of the VHT-SIG-A1 field and has a size of 2 bits. Ifthe 2 bits are set to 0, 1, 2, or 3, the BW field indicates 20 MHz, 40MHz, 80 MHz, or 160 and 80+80 MHz. For details of the fields included inthe VHT-SIG-A field, refer to the IEEE 802.11 ac-2013 technicalspecification. In the HE PPDU frame format of the present invention, theHE-SIG-A field may include one or more of the fields included in theVHT-SIG-A field, and it may provide backward compatibility with IEEE802.11ac stations.

FIG. 9 depicts subchannel allocation in the HE PPDU frame formataccording to the present invention.

In FIG. 9, it is assumed that information indicating subchannelsallocated to STAs in HE PPDU indicates that 0 MHz subchannel isallocated to STA1 (i.e., no subchannel is allocated), a 5-MHz subchannelis allocated to each of STA2 and STA3, and a 10-MHz subchannel isallocated to STA4.

In the example of FIG. 9, an L-STF, an L-LTF, an L-SIG, and a HE-SIG-Amay be transmitted per channel (e.g., 20 MHz), a HE-STF and a HE-LTF maybe transmitted on each of subchannels being basic subchannel units(e.g., 5 MHz), and a HE-SIG-B and a PSDU may be transmitted on each ofsubchannels allocated to STAs. A subchannel allocated to a STA has asize required for PSDU transmission to the STA. The size of thesubchannel allocated to the STA may be an N (N=1, 2, 3, . . . ) multipleof the size of the basic subchannel unit (i.e., a minimum-sizesubchannel unit). In the example of FIG. 9, the size of a subchannelallocated to STA2 is equal to that of the basic subchannel unit, thesize of a subchannel allocated to STA3 is equal to that of the basicsubchannel unit, and the size of a subchannel allocated to STA4 is twicelarger than that of the basic subchannel unit.

FIG. 9 illustrates a plurality of HE-LTF elements and a plurality ofHE-LTF subelements which are distinguished in the time and frequencydomains. One HE-LTF element may correspond to one OFDM symbol in thetime domain and one subchannel unit (i.e., the bandwidth of a subchannelallocated to a STA) in the frequency domain. One HE-LTF subelement maycorrespond to one OFDM symbol in the time domain and one basicsubchannel unit (e.g. 5 MHz) in the frequency domain. In the example ofFIG. 9, one HE-LTF element includes one HE-LTF subelement in the 5-MHzsubchannel allocated to STA2 or STA3. On the other hand, one HE-LTFelement includes two HE-LTF subelements in the third subchannel, i.e.,10-MHz subchannel, allocated to STA4. A HE-LTF element and a HE-LTFsubelement are logical units and the PHY layer does not always operatein units of a HE-LTF element or HE-LTF subelement.

A HE-LTF symbol may correspond to a set of HE-LTF elements in one OFDMsymbol in the time domain and one channel unit (e.g. 20 MHz) in thefrequency domain. That is, one HE-LTF symbol may be divided into HE-LTFelements by a subchannel width allocated to a STA and into HE-LTFsubelements by the width of the basic subchannel unit in the frequencydomain.

A HE-LTF section may correspond to a set of HE-LTF elements in one ormore OFDM symbols in the time domain and one subchannel unit (i.e. thebandwidth of a subchannel allocated to a STA) in the frequency domain. AHE-LTF subsection may correspond to a set of HE-LTF elements in one ormore OFDM symbols in the time domain and one basic subchannel unit(e.g., 5 MHz) in the frequency domain. In the example of FIG. 9, oneHE-LTF section includes one HE-LTF subsection in the 5-MHz subchannelallocated to STA2 or STA3. On the other hand, one HE-LTF sectionincludes two HE-LTF subsections in the third subchannel, i.e., 10-MHzsubchannel, allocated to STA4.

A HE-LTF field may correspond to a set of HE-LTF elements (orsubelements), HE-LTF symbols, or HE-LTF sections (or subsections) for aplurality of stations.

For the afore-described HE PPDU transmission, subchannels allocated to aplurality of HE STAs may be contiguous in the frequency domain. In otherwords, for HE PPDU transmission, the subchannels allocated to the HESTAs may be sequential and any intermediate one of the subchannels ofone channel (e.g., 20 MHz) may not be allowed to be unallocated orempty. Referring to FIG. 8, if one channel includes four subchannels, itmay not be allowed to keep the third subchannel unallocated and empty,while the first, second, and fourth subchannels are allocated to STAs.However, the present invention does not exclude non-allocation of aintermediate subchannel of one channel to a STA.

FIG. 10 depicts a subchannel allocation method according to the presentinvention.

In the example of FIG. 10, a plurality of contiguous channels (e.g.,20-MHz-bandwidth channels) and boundaries of the plurality of contiguouschannels are shown. In FIG. 10, a preamble may correspond to an L-STF,an L-LTF, an L-SIG, and a HE-SIG-A as illustrated in the examples ofFIGS. 8 and 9.

A subchannel for each HE STA may be allocated only within one channel,and may not be allocated with partially overlapping between a pluralityof channels. That is, if there are two contiguous 20-MHz channels CH1and CH2, subchannels for STAs paired for MU-MIMO-mode or OFDMA-modetransmission may be allocated either within CH1 or within CH2, and itmay be prohibited that one part of a subchannel exists in CH1 andanother part of the subchannel exists in CH2. This means that onesubchannel may not be allocated with crossing a channel boundary. Fromthe perspective of RUs supporting the MU-MIMO or OFDMA mode, a bandwidthof 20 MHz may be divided into one or more RUs, and a bandwidth of 40 MHzmay be divided into one or more RUs in each of two contiguous 20-MHzbandwidths, and no RU is allocated with crossing the boundary betweentwo contiguous 20-MHz bandwidths.

As described above, it is not allowed that one subchannel belongs to twoor more 20-MHz channels. Particularly, a 2.4-GHz OFDMA mode may supporta 20-MHz OFDMA mode and a 40-MHz OFDMA mode. In the 2.4-GHz OFDMA mode,it may not be allowed that one subchannel belongs to two or more 20-MHzchannels.

FIG. 10 is based on the assumption that subchannels each having the sizeof a basic subchannel unit (e.g., 5 MHz) in CH1 and CH2 are allocated toSTA1 to STA7, and subchannels each having double the size (e.g., 10 MHz)of the basic subchannel unit in CH4 and CH5 are allocated to STA8, STA9,and STA10.

As illustrated in the lower part of FIG. 9, although a subchannelallocated to STA1, STA2, STA3, STA5, STA6, or STA7 is fully overlappedonly with one channel (i.e., without crossing the channel boundary, orbelonging only to one channel), a subchannel allocated to STA4 ispartially overlapped with the two channels (i.e., crossing the channelboundary, or belonging to the two channels). In the forgoing example ofthe present invention, the subchannel allocation to STA4 is not allowed.

As illustrated in the upper part of FIG. 9, although a subchannelallocated to STA8 or STA10 is fully overlapped only with one channel(i.e., crossing the channel boundary, or belonging only to one channel),a subchannel allocated to STA9 is partially overlapped with two channels(i.e., crossing the channel boundary, or belonging to the two channels).In the forgoing example of the present invention, the subchannelallocation to STA9 is not allowed.

On the other hand, it may be allowed to allocate a subchannel partiallyoverlapped between a plurality of channels (i.e., crossing the channelboundary, or belonging to two channels). For example, in SU-MIMO modetransmission, a plurality of contiguous channels may be allocated to aSTA and any of one or more subchannels allocated to the STA may crossthe boundary between two contiguous channels.

While the following description is given with an assumption that onesubchannel has a channel bandwidth of 5 MHz in one channel having achannel bandwidth of 20 MHz, this is provided to simplify thedescription of the principle of the present invention and thus shouldnot be construed as limiting the present invention. For example, thebandwidths of a channel and a subchannel may be defined or allocated asvalues other than the above examples. In addition, a plurality ofsubchannels in one channel may have the same or different channelwidths.

FIG. 11 depicts the starting and ending points of a HE-LTF field in theHE PPDU frame format according to the present invention.

To support the MU-MIMO mode and the OFDMA mode, the HE PPDU frame formataccording to the present invention may include, in the HE-SIG-A field,information about the number of spatial streams to be transmitted to aHE STA allocated to each subchannel.

If MU-MIMO-mode or OFDMA-mode transmission is performed to a pluralityof HE STAs on one subchannel, the number of spatial streams to betransmitted to each of the HE STAs may be provided in the HE-SIG-A orHE-SIG-B field, which will be described later in detail.

FIG. 11 is based on the assumption that a first 5-MHz subchannel isallocated to STA1 and STA2 and two spatial streams are transmitted toeach STA in a DL MU-MIMO or OFDMA mode (i.e., a total of four spatialstreams are transmitted on one subchannel). For this purpose, a HE-STF,a HE-LTF, a HE-LTF, a HE-LTF, a HE-LTF, and a HE-SIG-B follow theHE-SIG-A field on the subchannel. The HE-STF is used for frequencyoffset estimation and phase offset estimation for the 5-MHz subchannel.The HE-LTFs are used for channel estimation for the 5-MHz subchannel.Since the subchannel carries four spatial streams, as many HE-LTFs(i.e., HE-LTF symbols or HE-LTF elements in a HE-LTF section) as thenumber of the spatial streams, that is, four HE-LTFs are required tosupport MU-MIMO transmission.

According to an example of the present invention, relationship between atotal number of spatial streams transmitted on one subchannel and anumber of HE-LTFs is listed in [Table 3].

TABLE 3 Total number of spatial streams Number of transmitted on onesubchannel HE-LTFs 1 1 2 2 3 4 4 4 5 6 6 6 7 8 8 8

Referring to [Table 3], if one spatial stream is transmitted on onesubchannel, at least one HE-LTF needs to be transmitted on thesubchannel. If an even number of spatial streams are transmitted on onesubchannel, at least as many HE-LTFs as the number of the spatialstreams need to be transmitted. If an odd number of spatial streamsgreater than one are transmitted on one subchannel, at least as manyHE-LTFs as a number of adding 1 to the number of the spatial streamsneed to be transmitted.

Referring to FIG. 11 again, it is assumed that the second 5-MHzsubchannel is allocated to STA3 and STA4 and one spatial streams per STAis transmitted in the DL MU-MIMO or OFDMA mode (i.e., a total of twospatial streams are transmitted on one subchannel). In this case, twoHE-LTFs need to be transmitted on the second subchannel, however, in theexample of FIG. 11, a HE-STF, a HE-LTF, a HE-LTF, a HE-LTF, a HE-LTF,and a HE-SIG-B follow the HE-SIG-A field on the subchannel (i.e., fourHE-LTFs are transmitted). This is for setting the same starting time ofPSDU transmission for subchannels allocated to other STAs paired withSTA3 and STA4 for MU-MIMO transmission. If only two HE-LTFs aretransmitted on the second subchannel, PSDUs are transmitted at differenttime points on the first and second subchannels. PSDU transmission oneach subchannel at a different time point results in discrepancy betweenOFDM symbol timings of subchannels, thereby no orthogonality ismaintained. To overcome this problem, an additional constraint need tobe imposed for HE-LTF transmission.

Basically, transmission of as many HE-LTFs as required is sufficient inan SU-MIMO or non-OFDMA mode. However, timing synchronization (oralignment) with fields transmitted on subchannels for other paired STAsis required in the MU-MIMO or OFDMA mode. Accordingly, the numbers ofHE-LTFs may be determined for all other subchannels based on asubchannel having the maximum number of streams in MU-MIMO-mode orOFDMA-mode transmission.

Specifically, the numbers of HE-LTFs may be determined for allsubchannels according to the maximum of the numbers of HE-LTFs (HE-LTFsymbols or HE-LTF elements in a HE-LTF section) required according tothe total numbers of spatial streams transmitted on each subchannel, fora set of HE STAs allocated to each subchannel. A “set of HE STAsallocated to each subchannel” is one HE STA in the SU-MIMO mode, and aset of HE STAs paired across a plurality of subchannels in the MU-MIMOmode. The ‘number of spatial streams transmitted on each subchannel’ isthe number of spatial streams transmitted to one HE STA in the SU-MIMOmode, and the number of spatial streams transmitted to a plurality of HESTAs paired on the subchannel in the MU-MIMO mode.

That is, it may be said that a HE-LTF field starts at the same timepoint and ends at the same time point in a HE PPDU for all users (i.e.HE STAs) in MU-MIMO-mode or OFDMA-mode transmission. Or it may be saidthat the lengths of HE-LTF sections are equal on a plurality ofsubchannels for all users (i.e. HE STAs) in MU-MIMO-mode or OFDMA-modetransmission. Or it may be said that the number of HE-LTF elementsincluded in each HE-LTF section is equal on a plurality of subchannelsfor all users (i.e. HE STAs) in MU-MIMO-mode or OFDMA-mode transmission.Accordingly, PSDU transmission timings may be synchronized among aplurality of subchannels for all HE STAs in MU-MIMO-mode or OFDMA-modetransmission.

As described above, the number of HE-LTF symbols (refer to FIG. 8) maybe 1, 2, 4, 6, or 8 in HE PPDU transmission in the MU-MIMO or OFDMAmode, determined according to the maximum of the numbers of spatialstreams on each of a plurality of subchannels. A different number ofspatial streams may be allocated to each of a plurality of subchannels,and the number of spatial streams allocated to one subchannel is thenumber of total spatial streams for all users allocated to thesubchannel. That is, the number of HE-LTF symbols may be determinedaccording to the number of spatial streams allocated to a subchannelhaving a maximum number of spatial streams by comparing the number oftotal spatial streams for all users allocated to one of a plurality ofsubchannels with the number of total spatial streams for all usersallocated to another subchannel.

Specifically, in HE PPDU transmission in the OFDMA mode, the number ofHE-LTF symbols may be 1, 2, 4, 6, or 8, determined based on the numberof spatial streams transmitted in a subchannel having a maximum numberof spatial streams across a plurality of subchannels. Further, in HEPPDU transmission in the OFDMA mode, the number of HE-LTF symbols may bedetermined based on whether the number of spatial streams transmitted ina subchannel having a maximum number of spatial streams across aplurality of subchannels is odd or even (refer to [Table 3]). That is,in HE PPDU transmission in the OFDMA mode, when the number (e.g., K) ofspatial streams transmitted in a subchannel having a maximum number ofspatial streams across a plurality of subchannels is an even number, thenumber of HE-LTF symbols may be equal to K. In HE PPDU transmission inthe OFDMA mode, when the number, K, of spatial streams transmitted in asubchannel having a maximum number of spatial streams across a pluralityof subchannels is an odd number greater than one, the number of HE-LTFsymbols may be equal to K+1.

When only one STA is allocated to one subchannel in OFDMA mode (i.e.,OFDMA mode without using MU-MIMO), a subchannel having a maximum numberof spatial streams across a plurality of subchannels may be determinedby the number of spatial streams for a STA allocated to each subchannel.When more than one STA is allocated to one subchannel in OFDMA mode(i.e., OFDMA mode using MU-MIMO), a subchannel having a maximum numberof spatial streams across a plurality of subchannels may be determinedby the number of STAs allocated to each subchannel and the number ofspatial streams for each STA allocated to each subchannel (e.g., if STA1and STA2 are allocated to one subchannel, sum of the number of spatialstreams for STA1 and the number of spatial streams for STA2).

When transmitting a HE PPDU frame in the MU-MIMO or OFDMA mode, atransmitter may generate P (P is an integer equal to or larger than 1)HE-LTF symbols (refer to FIG. 8) and transmit a HE PPDU frame includingat least the P HE-LTF symbols and a Data field to a receiver. The HEPPDU frame may be divided into Q subchannels in the frequency domain (Qis an integer equal to or larger than 2). Each of the P HE-LTF symbolsmay be divided into Q HE-LTF elements corresponding to the Q subchannelsin the frequency domain. That is, the HE PPDU may include P HE-LTFelements on one subchannel (herein, the P HE-LTF elements may belong toone HE-LTF section on the subchannel).

As described above, the number of HE-LTF elements (i.e., P) in one ofthe Q subchannels may be equal to the number of HE-LTF elements (i.e. P)of another subchannel. Also, the number of HE-LTF elements (i.e., P)included in a HE-LTF section in one of the Q subchannels may be equal tothe number of HE-LTF elements (i.e. P) included in a HE-LTF section inanother subchannel. The HE-LTF section of one of the Q subchannels maystart and end at the same time points as the HE-LTF section of anothersubchannel. Also, the HE-LTF sections may start and end at the same timepoints across the Q subchannels (i.e., across all users or stations).

Referring to FIG. 11 again, the third 5-MHz subchannel is allocated toSTA5 and one spatial stream is transmitted on the subchannel in SU-MIMO(considering all subchannels, a plurality of spatial streams aretransmitted to STA1 to STA6 in MU-MIMO or OFDMA mode). In this case,although transmission of one HE-LTF is sufficient for the subchannel, asmany HE-LTFs as the maximum of the numbers of HE-LTFs on the othersubchannels, that is, four HE-LTFs are transmitted on the subchannel inorder to align the starting points and ending points of the HE-LTFfields of the subchannels.

The fourth 5-MHz subchannel is allocated to STA6 and one spatial streamis transmitted on the subchannel in SU-MIMO (considering all othersubchannels, a plurality of spatial streams are transmitted to STA1 toSTA6 in MU-MIMO or OFDMA mode). In this case, although transmission ofone HE-LTF is sufficient for the subchannel, as many HE-LTFs as themaximum of the numbers of HE-LTFs on the other subchannels, that is,four HE-LTFs are transmitted on the subchannel in order to align thestarting points and ending points of the HE-LTF fields of thesubchannels.

In the example of FIG. 11, the remaining two HE-LTFs except two HE-LTFsrequired for channel estimation of STA3 and STA4 on the secondsubchannel, the remaining three HE-LTFs except one HE-LTF required forchannel estimation of STA5 on the third subchannel, and the remainingthree HE-LTFs except one HE-LTF required for channel estimation of STA6on the fourth subchannel may be said to be placeholders that areactually not used for channel estimation at the STAs.

FIG. 12 depicts a HE-SIG-B field and a HE-SIG-C field in the HE PPDUframe format according to the present invention.

To effectively support MU-MIMO-mode or OFDMA-mode transmission in the HEPPDU frame format according to the present invention, independentsignaling information may be transmitted on each subchannel.Specifically, a different number of spatial streams may be transmittedto each of a plurality of HE STAs that receive an MU-MIMO-mode orOFDMA-mode transmission simultaneously. Therefore, information about thenumber of spatial streams to be transmitted should be indicated to eachHE STA.

Information about the number of spatial streams on one channel may beincluded in, for example, a HE-SIG-A field. A HE-SIG-B field may includespatial stream allocation information about one subchannel. Also, aHE-SIG-C field may be transmitted after transmission of HE-LTFs,including MCS information about a PSDU and information about the lengthof the PSDU, etc.

FIG. 13 depicts OFDM symbol durations and GI lengths in the HE PPDUframe format according to the present invention.

In the HE PPDU frame format according to the present invention, L-STF,L-LTF, L-SIG, and HE-SIG-A fields may be configured with 4.0-μs OFDMsymbols based on 64-FFT. One OFDM symbol has a GI of 0.8 μs. In thepresent invention, a GI value applied to the L-STF, L-LTF, L-SIG, andHE-SIG-A fields is defined as G1. The L-STF, L-LTF, L-SIG, and HE-SIG-Afields may include 3.2-μs OFDM symbols based on 64-FFT, excluding theGIs. The term 64 FFT-based symbol is used mainly based on a channelbandwidth of 20 MHz. If the term 64 FFT-based symbol is usedirrespective of a channel bandwidth, a 64 FFT-based symbol may mean asymbol having a symbol duration of 3.2 μs and a subcarrier spacing of312.5 kHz.

The following HE-STF, HE-LTF, HE-SIG-B, and PSDU fields may include16-μs OFDM symbols based on 256-FFT. The OFDM symbol duration may bechanged according to a GI value. Two types of GI values may be definedfor one OFDM symbol during different time periods. A GI value applied tothe OFDM symbols of the HE-STF, HE-LTF, and HE-SIG-B fields is definedas G2 and a GI value applied to the OFDM symbols of the PSDU is definedas G3. Excluding the GIs, the HE-STF, HE-LTF, HE-SIG-B, and PSDU fieldsmay be configured with 12.8-μs OFDM symbols based on 256-FFT. The term256 FFT-based symbol is used mainly based on a channel bandwidth of 20MHz. If the term 256 FFT-based symbol is used irrespective of a channelbandwidth, a 256 FFT-based symbol may mean a symbol having a symbolduration of 12.8 μs and a subcarrier spacing of 78.125 kHz.

The values of G2 and G3 may be equal or different. If G2 and G3 areequal, G2 and G3 may be defined as one parameter without distinguishingbetween G2 and G3. Unlike G1, G2 and G3 may vary according to atransmitted PPDU transmission vector, rather than they are fixed values(i.e., predetermined values). This is related to that the lengths of theHE-STF, HE-LTF, and HE-SIG-B fields to which G2 is applied may varyaccording to a PPDU transmission vector and the length of the PSDU towhich G3 is applied may also vary according to the PPDU transmissionvector.

For example, G1 may have a fixed value (i.e., a predetermined value) of0.8 μs, G2 may be a value selected from 3.2 μs, 1.6 μs, 0.8 μs, and 0.4μs, and G3 may be a value selected from among 3.2 μs, 1.6 μs, 0.8 μs,and 0.4 μs. Also, G1 may have a fixed value (i.e., a predeterminedvalue) of 0.8 μs, and G2 or G3 may be a value selected or determinedfrom among 3.2 μs, 1.6 μs, 0.8 μs, and 0.4 μs. G1 does not requireseparate signaling because G1 is a fixed value, and signalinginformation indicating G2 and G3 may be provided in the HE-SIG-A fieldto a HE STA.

G2 and G3 are applied commonly across all OFDM symbols transmittedduring a corresponding time period and across all subchannels.Accordingly, PSDU transmission timings and OFDM symbol timings may besynchronized. For example, it is not allowed to apply a 3.2-μs G2 valueto a subchannel and a 1.6-μs or 0.8-μs G2 value to another subchannelduring a specific time period. Rather, the same 3.2-μs G2 value may beapplied to the subchannels during the same time period. In a similarexample, it is not allowed to apply a 1.6-μs G3 value to a subchanneland a 3.2-μs or 0.8-μs G3 value to another subchannel during a specifictime period. Rather, the same 1.6-μs G3 value may be applied to thesubchannels during the same time period.

In the case where a HE PPDU frame format having HE-LTF sections ofdifferent lengths for subchannels is used (i.e., in the case where thenumber of HE-LTFs is not determined for each subchannel based on themaximum of the numbers of HE-LTFs required according to the total numberof spatial streams transmitted on each subchannel in a set of HE STAsallocated to each of subchannels, as described in the example of FIG.11), if the values of G2 and G3 are different, PSDUs are transmitted ondifferent subchannels at different time points and OFDM symbol timingsare not synchronized. Therefore, values of G2 and G3 may need to beselected or determined as a same value.

In the case where a HE PPDU frame format having HE-LTF sections of thesame length for subchannels is used (i.e., in the case where the numberof HE-LTFs is determined for each subchannel based on the maximum of thenumbers of HE-LTFs required according to the total number of spatialstreams transmitted on each subchannel in a set of HE STAs allocated toeach of subchannels, as described in the example of FIG. 11), eventhough the values of G2 and G3 are different, PSDUs are transmitted onthe subchannels at the same time point, without causing discrepancybetween OFDM symbol timings. Therefore, values of G2 and G3 may beselected or determined as different values. However, even in this case,the present invention does not exclude that values of G2 and G3 may beselected or determined as a same value.

In the example of FIG. 3, OFDM symbol durations S1, S2, and S3 may beapplied respectively to time periods to which the GIs G1, G2, and G3 areapplied.

FIG. 14 depicts an exemplary HE PPDU frame format for a wide channelband according to the present invention.

Referring to FIG. 14, the HE PPDU frame format for one 20-MHz channelillustrated in the example of FIG. 13 is extended to two 20-MHzchannels. Similarly, HE PPDU frame formats for the channel bandwidths of80 MHz and 160 MHz may be configured by extending the HE PPDU frameformat for one 20-MHz channel illustrated in the example of FIG. 13 tofour and eight 20-MHz channels, respectively.

There is no modification involved in extending the HE PPDU frame formatfor one 20-MHz channel. In other words, all subchannels across one ormore 20-MHz channels are the same in terms of PSDU transmission timepoints, OFDM symbol durations, and GIs.

From this viewpoint, the example described with reference to FIG. 11 inwhich “the lengths of HE-LTF sections across subchannels are equal” maybe extended to simultaneous application on a channel basis as well as ona subchannel basis. Therefore, PSDU transmission timings and OFDM symboltimings are synchronized for users paired for MU-MIMO-mode or OFDMA-modetransmission, thus maintaining orthogonality. This channel-based examplewill be described below.

Basically in SU-MIMO-mode or non-OFDMA-mode transmission, it issufficient to transmit as many HE-LTFs as required. However, the timingsof fields transmitted on subchannels for other paired STAs need to besynchronized (or aligned) across all subchannels over one or more 20-MHzchannels in MU-MIMO-mode or OFDMA-mode transmission. Therefore, thenumbers of HE-LTFs on all other subchannels over one or more 20-MHzchannels may be determined based on a subchannel having a maximum numberof streams among all subchannels over one or more 20-MHz channels inMU-MIMO-mode or OFDMA-mode transmission.

Specifically, the numbers of HE-LTFs to be transmitted on allsubchannels may be determined according to the maximum of the numbers ofHE-LTFs required according to the total numbers of spatial streamstransmitted on each subchannels over one or more 20-MHz channels, for aset of HE STAs allocated to each of the subchannels. Herein, ‘the set ofHE STAs allocated to each of the subchannels over one or more 20-MHzchannels’ is one HE STA in the SU-MIMO mode, whereas it is a set of aplurality of HE STAs paired on all subchannels over one or more 20-MHzchannels in the MU-MIMO mode or OFDMA mode. The ‘total number of spatialstreams transmitted on each of all subchannels over one or more 20-MHzchannels’ is the number of spatial streams transmitted to one HE STA inthe SU-MIMO mode and the number of spatial streams transmitted to aplurality of HE STAs paired on the subchannel in the MU-MIMO mode orOFDMA mode.

That is, it may be said that a HE-LTF field starts at the same timepoint and ends at the same time point on all subchannels over one ormore 20-MHz channels for all users (i.e., HE STAs) in MU-MIMO-mode orOFDMA-mode transmission of a HE PPDU. Or it may be said that the lengthsof HE-LTF sections are equal on all subchannels over one or more 20-MHzchannels for all HE STAs in MU-MIMO-mode or OFDMA-mode transmission. Orit may be said that the number of HE-LTF elements included in eachHE-LTF section is equal in all subchannels over one or more 20-MHzchannels for all HE STAs in MU-MIMO-mode or OFDMA-mode transmission.Accordingly, PSDU transmission timings may be synchronized between allsubchannels over one or more 20-MHz channels for all HE STAs inMU-MIMO-mode or OFDMA-mode transmission.

In FIG. 14, the OFDM symbol duration and GI of L-STF, L-LTF, L-SIG, andHE-SIG-A fields on the first 20-MHz subchannel are S1 and G1,respectively. Like the first 20-MHz subchannel, the first 20-MHzsubchannel has S1 and G1 respectively as the OFDM symbol duration and GIof L-STF, L-LTF, L-SIG, and HE-SIG-A fields.

In FIG. 14, a HE-STF field, a plurality of HE-LTFs, and a HE-SIG-B fieldon the first 20-MHz channel are S2 and G2, respectively. Like the first20-MHz channel, the OFDM symbol duration and GI of a HE-STF field, aplurality of HE-LTFs, and a HE-SIG-B field on the second 20-MHz channelare also S2 and G2, respectively.

In FIG. 14, the OFDM symbol duration and GI of a PSDU on the first20-MHz channel are S3 and G3, respectively. Like the first 20-MHzchannel, the OFDM symbol duration and GI of a PSDU on the second 20-MHzchannel are also S3 and G3, respectively.

This example means that if the OFDM symbol duration and GI of one 20-MHzchannel are determined based on 64-FFT, the OFDM symbol duration and GIof the other 20-MHz channel(s) should be determined based on 64-FFT. Inother words, if the OFDM symbol duration and GI of one 20-MHz channelare determined based on 64-FFT, the OFDM symbol duration and GI of theother 20-MHz channel(s) should not be determined based on 256-FFT.

In a modification example, although subchannels within one 20-MHzchannel may have the same OFDM symbol durations and the same GIs,subchannels within another 20-MHz channel may have different OFDM symboldurations and GIs. For example, while S2, G2, S3, and G3 are applied asOFDM symbol durations and GIs for subchannels within the first 20-MHzchannel, different values (e.g., S4, G4, S5, and G5) may be applied asOFDM symbol durations and GIs for subchannels within the second 20-MHzchannel. Even in this case, the OFDM symbol duration and GI, S1 and G1,applied to L-STF, L-LTF, and L-SIG fields in a different 20-MHz channelare the same fixed values in every 20-MHz channel.

Further, this modification example may include application of theexample described before with reference to FIG. 11 in which ‘subchannelshave the same HE-LTF section length’ only to subchannels within one20-MHz channel, not to the HE-LTF section length of subchannels inanother 20-MHz channel.

With reference to the foregoing examples of the present invention,mainly the features of a HE PPDU frame structure applicable to a DLMU-MIMO-mode or OFDMA-mode transmission that an AP transmitssimultaneously to a plurality of STAs have been described. Now, adescription will be given of the features of a HE PPDU frame structureapplicable to a UL MU-MIMO-mode or OFDMA-mode transmission that aplurality of STAs transmits simultaneously to an AP.

The above-described various examples of structures of the HE PPDU frameformat supporting MU-MIMO-mode or OFDMA-mode transmission should not beunderstood as applicable only to DL without applicable UL. Rather, theexamples should be understood as also applicable to UL. For example, theabove-described exemplary HE PPDU frame formats may also be used for aUL HE PPDU transmission that a plurality of STAs simultaneouslytransmits to a single AP.

However, in the case of a DL MU-MIMO-mode or OFDMA-mode HE PPDUtransmission that an AP simultaneously transmits to a plurality of STAs,the transmission entity, AP has knowledge of the number of spatialstreams transmitted to a HE STA allocated to each of a plurality ofsubchannels. Therefore, the AP may include, in a HE-SIG-A field or aHE-SIG-B field, information about the total number of spatial streamstransmitted across a channel, a maximum number of spatial streams (i.e.,information being a basis of the number of HE-LTF elements (or thestarting point and ending point of a HE-LTF section) on eachsubchannel), and the number of spatial streams transmitted on eachsubchannel. In contrast, in the case of a UL MU-MIMO-mode or OFDMA-modeHE PPDU transmission that a plurality of STAs simultaneously transmitsto an AP, each STA being a transmission entity may be aware only of thenumber of spatial streams in a HE PSDU that it will transmit, withoutknowledge of the number of spatial streams in a HE PSDU transmitted byanother STA paired with the STA. Accordingly, the STA may determineneither the total number of spatial streams transmitted across a channelnor a maximum number of spatial streams.

To solve this problem, a common parameter (i.e., a parameter appliedcommonly to STAs) and an individual parameter (a separate parameterapplied to an individual STA) may be configured as follows in relationto a UL HE PPDU transmission.

For simultaneous UL HE PPDU transmissions from a plurality of STAs to anAP, a protocol may be designed in such a manner that the AP sets acommon parameter or individual parameters (common/individual parameters)for the STAs for the UL HE PPDU transmissions and each STA operatesaccording to the common/individual parameters. For example, the AP maytransmit a trigger frame (or polling frame) for a UL MU-MIMO-mode orOFDMA-mode transmission to a plurality of STAs. The trigger frame mayinclude a common parameter (e.g., the number of spatial streams across achannel or a maximum number of spatial streams) and individualparameters (e.g., the number of spatial streams allocated to eachsubchannel), for the UL MU-MIMO-mode or OFDMA-mode transmission. As aconsequence, a HE PPDU frame format applicable to a UL MU-MIMO or OFDMAmode may be configured without a modification to an exemplary HE PPDUframe format applied to a DL MU-MIMO or OFDMA mode. For example, eachSTA may configure a HE PPDU frame format by including information aboutthe number of spatial streams across a channel in a HE-SIG-A field,determining the number of HE-LTF elements (or the starting point andending point of a HE-LTE section) on each subchannel according to themaximum number of spatial streams, and including information about thenumber of spatial streams for the individual STA in a HE-SIG-B field.

Alternatively, if the STAs operate always according to thecommon/individual parameters received in the trigger frame from the AP,each STA does not need to indicate the common/individual parameters tothe AP during a HE PPDU transmission. Therefore, this information maynot be included in a HE PPDU. For example, each STA may have only todetermine the total number of spatial streams, the maximum number ofspatial streams, and the number of spatial streams allocated toindividual STA, as indicated by the AP, and configure a HE PPDUaccording to the determined numbers, without including information aboutthe total number of spatial streams or the number of spatial streamsallocated to the STA in the HE PPDU.

On the other hand, if the AP does not provide common/individualparameters in a trigger frame, for a UL MIMO-mode or OFDMA-mode HE PPDUtransmission, the following operation may be performed.

Common transmission parameters (e.g., channel BandWidth (BW)information, etc.) for simultaneously transmitted HE PSDUs may beincluded in HE-SIG-A field, but parameters that may be different forindividual STAs (e.g., the number of spatial streams, an MCS, andwhether STBC is used or not, for each individual STA) may not beincluded in HE-SIG-A field. Although the individual parameters may beincluded in HE-SIG-B field, information about the number of spatialstreams and information indicating whether STBC is used or not, need tobe transmitted before a HE-LTF field because the number of spatialstreams and the information indicating whether STBC is used or not aresignificant to determination of configuration information about apreamble and a PSDU in a HE PPDU frame format (e.g., the number ofHE-LTF elements is determined according to a combination of the numberof spatial streams and the information indicating whether STBC is usedor not). For this purpose, a HE PPDU frame format as illustrated in FIG.15 may be used for a UL HE PPDU transmission.

FIG. 15 depicts another exemplary HE PPDU frame format according to thepresent invention. The HE PPDU frame format illustrated in FIG. 15 ischaracterized in that a structure of HE-SIG-A, HE-SIG-B, and HE-SIG-Cfields similar to in FIG. 11 is used for a UL PPDU transmission.

As described before, if a UL MU-MIMO-mode or OFDMA-mode transmission isperformed by triggering of an AP (according to common/individualparameters provided by the AP), an individual STA may not need to reportan individual parameter to the AP. In this case, one or more of aHE-SIG-B field, a HE-SIG-C field, and a first HE-LTF element (i.e., aHE-LTF between a HE-STF field and a HE-SIG-B field) illustrated in FIG.15 may not exist. In this case, a description of each field given belowmay be applied only in the presence of the field.

In the example of FIG. 15, a HE-SIG-A field is transmitted per channel(i.e., per 20-MHz channel) and may include transmission parameterscommon to simultaneously transmitted HE PSDUs. Since the sameinformation is transmitted in up to HE-SIG-A fields in UL PPDUstransmitted by HE STAs allocated to subchannels, the AP may receive thesame signals from the plurality of STAs successfully.

A HE-SIG-B field is transmitted per subchannel in one channel. TheHE-SIG-B field may have an independent parameter value according to thetransmission characteristics of a HE PSDU transmitted on eachsubchannel. The HE-SIG-B field may include spatial stream allocationinformation and information indicating whether STBC is used or not, foreach subchannel. If MU-MIMO is applied to a subchannel (i.e., if aplurality of STAs perform transmission on a subchannel), the HE-SIG-Bfield may include a common parameter for the plurality of STAs paired onthe subchannel.

A HE-SIG-C field is transmitted on the same subchannel as the HE-SIG-Bfield and may include information about an MCS and a packet length. IfMU-MIMO is applied to a subchannel (i.e., if a plurality of STAs performtransmission on a subchannel), the HE-SIG-C field may include respectiveindividual parameters for each of the plurality of STAs paired on thesubchannel.

Similarly to DL MU-MIMO-mode or OFDMA-mode HE PPDU transmission,transmissions of PSDUs may start at different time points on subchannelsin UL MU-MIMO-mode or OFDMA-mode HE PPDU transmission, and if OFDMsymbols are not aligned accordingly, then the implementation complexityof an AP that receives a plurality of PSDUs increased. To solve thisproblem, ‘the number of HE-LTFs may be determined for all subchannelsaccording to the maximum of the numbers of HE LTFs required according tothe total numbers of spatial streams transmitted on each subchannel fora set of HE STAs allocated to each of subchannels’ as described withreference to the example of FIG. 11.

This feature may mean that the HE-LTF field start at the same time pointand end at the same time point across all users (i.e., HE STAs) in ULMU-MIMO-mode or OFDMA-mode transmission. Or it may be said that theHE-LTF sections of a plurality of subchannels have the same lengthacross all HE STAs in UL MU-MIMO-mode or OFDMA-mode transmission. Or itmay be said that each of the HE-LTF sections of a plurality ofsubchannels includes the same number of HE-LTF elements across all HESTAs in UL MU-MIMO-mode or OFDMA-mode transmission. Therefore, PSDUtransmission timings are synchronized between a plurality of subchannelsacross all HE STAs in UL MU-MIMO-mode or OFDMA-mode transmission.

In the HE PPDU frame format supporting UL MIMO-mode or OFDMA-modetransmission illustrated in FIG. 15, the L-STF, L-LTF, L-SIG, andHE-SIG-A fields may include 4.0-μs OFDM symbols based on 64-FFT. OneOFDM symbol has a GI of 0.8 μs. In the present description, A GI valueapplied to the L-STF, L-LTF, L-SIG, and HE-SIG-A fields is defined asG1. Excluding the GI, the L-STF, L-LTF, L-SIG, and HE-SIG-A fields maybe configured as 3.2-μs OFDM symbols based on 64-FFT.

In the example of FIG. 15, a HE-STF field, a HE-LTF field, a HE-SIG-Bfield, HE-LTF elements(s) in a HE-LTF section, HE-SIG-C field and a PSDUmay include 16-μs OFDM symbols based on 256-FFT. The OFDM symbolduration may be changed according to a GI value. Two types of GI valuesmay be defined for one OFDM symbol for different time periods. A GIvalue applied to the OFDM symbols of the HE-STF field, the HE-LTF field,the HE-SIG-B field, the HE-LTF elements(s) in the HE-LTF section, andthe HE-SIG-C field is defined as G2 and a GI value applied to the OFDMsymbols of the PSDU is defined as G3. Excluding the GIs, the HE-STFfield, the HE-LTF field, the HE-SIG-B field, and the PSDU may include12.8-μs OFDM symbols based on 256-FFT.

The values of G2 and G3 may be equal or different. If G2 and G3 areequal, G2 and G3 may be defined as one parameter without distinguishingG2 from G3. Unlike G1, G2 and G3 may vary according to each transmittedPPDU transmission vector, rather than they are fixed values (i.e.predetermined values known to both a transmitter and a receiver). Thisis related to the fact that the lengths of the HE-STF, the HE-LTF, theHE-SIG-B, the HE-LTF element(s) in a HE-LTF section, and the HE-SIG-C towhich G2 is applied may vary according to a PPDU transmission vector andthe length of the PSDU to which G3 is applied may also vary according tothe PPDU transmission vector.

In another example, the G1 applied to the L-STF, L-LTF, L-SIG, andHE-SIG-A fields (to which 64-FFT is applied) may be a fixed value (i.e.,a predefined value known to both a transmitter and a receiver) and oneof G2 and G3 (if G2 and G3 are equal, they may be defined as oneparameter) applied to the following fields (i.e., the HE-STF, HE-LTF,HE-SIG-B, HE-SIG-C, and PSDU to which 256-FFT is applied) may beconfigured or indicated as a variable value (e.g., one of 3.2 μs, 1.6μs, 0.8 μs, and 0.4 μs).

More specifically, G1 may have a fixed value (i.e. a predefined valueknown to both a transmitter and a receiver) of 0.8 μs, G2 may be a valueselected or indicated from among 3.2 μs, 1.6 μs, 0.8 μs, and 0.4 μs, andG3 may be a value selected or indicated from among 3.2 μs, 1.6 μs, 0.8μs, and 0.4 μs. Also, G1 may be a fixed value (i.e. a predefined valueknown to both a transmitter and a receiver) of 0.8 μs, and G2 or G3 maybe a value selected or indicated from among 3.2 μs, 1.6 μs, 0.8 μs, and0.4 μs. G1 does not require signaling because G1 is a fixed value, andsignaling information indicating G2 and G3 may be provided to the AP. Ifa HE STA performs UL transmission according to triggering of the AP (orbased on parameters provided by the AP), the HE-STA does not need toindicate the value of G2 or G3 to the AP.

G2 and G3 are applied commonly across all OFDM symbols transmittedduring a corresponding time period and across all subchannels.Accordingly, PSDU transmission timings may be synchronized, and OFDMsymbol timings may be synchronized. For example, it is not allowed that,if a 3.2-μs G2 value is applied to a subchannel during a specific timeperiod, a 1.6-μs or 0.8-μs G2 value is applied to other subchannelsduring the same time period. Rather, the same 3.2-μs G2 value may beapplied to other subchannels during the same time period. In a similarexample, it is not allowed that, if a 1.6-μs G3 value is applied to asubchannel during a specific time period, a 3.2-μs or 0.8-μs G3 value isapplied to other subchannels during the same time period. Rather, thesame 1.6-μs G3 value may be applied to other subchannels during the sametime period.

In the case where a HE PPDU frame format having HE-LTF sections ofdifferent lengths for subchannels is used (i.e., in the case where ‘thenumber of HE-LTFs is not determined for each subchannel based on themaximum of the numbers of HE-LTFs required according to the totalnumbers of spatial streams transmitted on subchannels in a set of HESTAs allocated to each of the subchannels’), if the values of G2 and G3are different, a PSDU is transmitted on each subchannel at a differenttime point and OFDM symbol timings are not synchronized. Therefore, thesame values may need to be selected or indicated as G2 and G3 in thiscase.

In the case where a HE PPDU frame format having HE-LTF sections of thesame length for subchannels is used (i.e., in the case where ‘the numberof HE-LTFs is determined for each subchannel based on the maximum of thenumbers of HE-LTFs required according to the total numbers of spatialstreams transmitted on subchannels in a set of HE STAs allocated to eachof the subchannels’), even though the values of G2 and G3 are different,PSDUs are transmitted on the subchannels at the same time point, withoutcausing discrepancy between OFDM symbol timings. Therefore, selection orindication of different values as G2 and G3 does not cause a problem.However, even in this case, selection or indication of the same valuesas G2 and G3 is not excluded.

In the example of FIG. 15, OFDM symbol durations S1, S2, and S3 may beapplied respectively to time periods to which the GIs G1, G2, and G3 areapplied.

As described before, a plurality of STAs may simultaneously transmitPSDUs in a HE PPDU frame format on their allocated subchannels or ontheir allocated spatial streams to an AP (i.e., referred to as ULMU-MIMO or OFDMA transmission or “UL MU transmission”) and maysimultaneously receive PSDUs in the HE PPDU frame format on theirallocated subchannels on their allocated spatial streams from the AP(i.e., referred to as DL MU-MIMO or OFDMA transmission or “DL MUtransmission”).

Hereinafter, examples of the present invention for applying beamformingand acquiring Channel State Information (CSI) in HE PPDU transmissionfor DL/UL MU transmission will be described. Beamforming transmissionmay mean a transmission scheme for controlling directions of beams toeach STA and the CSI may include channel state information needed tocalculate a beamforming steering matrix. Specifically, a descriptionwill be given of a HE PPDU frame configuration when beamforming isapplied for DL/UL MU transmission, a procedure for a beamformer (i.e., atransmitter of a beamformed signal) to acquire the CSI from a beamformee(i.e., a receiver of the beamformed signal), and a frame configurationused for the CSI acquisition procedure.

According to the present invention, in DL MU transmission or UL MUtransmission, whether beamforming is applied to a plurality ofsubchannels (or a plurality of STAs) in a HE PPDU transmitted over onetransmission channel divided into the plurality of subchannels may beidentically or differently configured.

In addition, information indicating whether beamforming is applied toeach subchannel (or each STA) may be included in a HE-SIG field in a HEPPDU format supporting beamforming in DL MU transmission or UL MUtransmission. Further, information directly or indirectly indicatingwhether beamforming is applied to each subchannel (or each STA) may beincluded in a trigger frame triggering UL MU transmission in order tosupport beamforming in UL MU transmission.

According to the present invention, an AP may simultaneously receive CSIfrom a plurality of STAs for beamformed DL MU transmission. For the APto acquire the CSI from the multiple STAs, an NDP sounding procedure maybe performed. The AP may determine whether beamforming is applied to themultiple STAs (or multiple subchannels allocated to the multiple STAs)based on the CSI acquired from the multiple STAs.

Hereinafter, various embodiments of the present invention forbeamforming support in a HE PPDU will be described in more detail.

FIGS. 16 and 17 depict exemplary HE PPDU frame formats supportingbeamforming according to the present invention.

In the example of FIG. 16, an AP corresponding to a beamformer transmitsPSDUs over subchannels allocated respectively to STA1, STA2, STA3, andSTA4 corresponding to beamformees. Signaling information indicatingwhether beamforming is applied to a PSDU over each subchannel may beincluded in a HE-SIG-A field of a HE PPDU.

Whether beamforming is applied to each of multiple subchannels in onechannel may be individually or independently determined. That is,beamforming may be identically or differently applied to multiplesubchannels (or multiple STAs). In this case, signaling bits having thesame size as the number of subchannels are needed. For example, whetherbeamforming is applied to each of N subchannels may be indicated byN-bit signaling information and a bit value (i.e., 0 or 1) of a bitposition of each of N bits may indicate that beamforming is not appliedor is applied to one subchannel.

Alternatively, whether beamforming is applied to a plurality ofsubchannels in one channel may be commonly determined. That is,beamforming may be identically applied or may not be identically appliedto all the subchannels. In this case, the signaling information may havea size of one bit.

In the example of FIG. 16, an AP corresponding to a beamformer transmitsPSDUs over subchannels allocated respectively to STA1, STA2, STA3, andSTA4 corresponding to beamformees. Signaling information indicatingwhether beamforming is allocated to a PSDU over each subchannel may beincluded in a HE-SIG-B field of a HE PPDU.

If the HE-SIG-B field is transmitted over each subchannel, 1-bitsignaling information indicating whether beamforming is applied to acorresponding subchannel may be included in one HE-SIG-B field. Whetherbeamforming is applied to each of multiple subchannels may beindividually or independently determined.

If the HE-SIG-B field is commonly transmitted over the multiplesubchannels, N-bit signaling information individually indicating whetherbeamforming is applied to N subchannels or 1-bit signaling informationcommonly indicating whether beamforming is applied to the N subchannelsmay be included in the HE-SIG-B field.

In this way, the signaling information indicating whether beamforming isapplied to the multiple subchannels may be included in a HE-SIG field(e.g., the HE-SIG-A or HE-SIG-B field). More specifically, the signalinginformation indicating whether beamforming is applied to the multiplesubchannels may be included in the HE-SIG field transmitted prior to aHE-LTF field. A HE-LTF section in each subchannel is used for channelestimation of the corresponding subchannel and channel estimation mayvary according to whether beamforming is applied to the subchannel.Therefore, the information indicating whether beamforming is applied maybe transmitted at least before the HE-LTF field (or before a HE-STFfield).

In addition, in beamformed HE PPDU transmission, a beamforming steeringmatrix may be applied to a transmission signal waveform and, in thiscase, smoothing may not be applied. Smoothing refers to acquisition ofnoise suppression gain in consideration of a neighboring subcarriersignal in channel estimation, using a high channel correlation betweenadjacent subcarriers in a channel environment in which delay spread islow. In beamformed HE PPDU transmission, since a channel correlationbetween adjacent subcarriers may not be maintained, channel estimationperformed by reflecting an adjacent subcarrier signal through smoothingmay not necessarily ensure noise suppression. Specifically, if whetherbeamforming is applied to each of multiple subchannels in beamformed HEPPDU transmission is individually or independently determined,subchannels to which beamforming is not applied and subchannels to whichbeamforming is applied may be adjacent to each other and, in this case,smoothing may not be applied to adjacent subcarriers over a subchannelboundary.

Thus, a certain STA may not apply smoothing to subcarrier(s) located ata boundary of subcarriers allocated thereto. For example, in the exampleof FIG. 16 or 17, if STA2 is not aware of whether beamforming is appliedto other subchannels (e.g., a subchannel allocated to STA1 and asubchannel allocated to STA3) adjacent to a subchannel allocated toSTA2, STA2 may not apply smoothing to subcarriers located at asubcarrier boundary (i.e., subcarriers adjacent to the subchannelallocated to STA1 and subcarriers adjacent to the subchannel allocatedto STA3) in the subchannel allocated to STA2.

FIG. 18 depicts an exemplary sounding protocol according to the presentinvention.

A sounding protocol is a procedure in which an AP acquires feedbackinformation about CSI between the AP and an STA from the STA in orderfor the AP to perform beamforming transmission to the STA. Through thesounding procedure, the AP may transmit an NDP frame (i.e., a PPDU framehaving a PSDU of length 0) to the STA and the STA may determine the CSIbased on the NDP frame and transmit a determined result (i.e., feedbackinformation) to the AP. The CSI determined based on the NDP frame may befed back to the AP that has requested sounding through a compressedbeamforming frame. The NDP frame transmitted by the AP may be receivedby a plurality of STAs and the AP may acquire the feedback informationfrom each STA.

Prior to transmission of the NDP frame by the AP, an NDP Announcement(NDPA) frame may be transmitted. The NDPA frame is a PPDU frameindicating that the NDP frame immediately follows the NDPA frame (e.g.,that the NDP frame will be transmitted after an SIFS from an end timepoint of the NDPA frame). The first STA of an STA list included in theNDPA frame may receive the NDP frame even without receiving anadditional polling frame (e.g. a Beamforming Report Polling frame) fromthe AP and then immediately transmit a feedback frame to the AP (e.g.,after an SIFS from a reception end time point of the NDP frame). Theother STAs included in the NDPA frame may receive the polling frame(e.g., the Beamforming Report Poll frame) from the AP and thenimmediately transmit the feedback frame to the AP (e.g., after an SIFSfrom a reception end time point of the polling frame). Herein, thepolling frame may be transmitted from the AP to an STA immediately aftera compressed beamforming frame from the STA is ended (e.g., after anSIFS from an end time point of the compressed beamforming frame).

The NDPA frame should be transmitted so as to be received by target STAsof the sounding procedure. Therefore, beamforming may not be applied totransmission of the NDPA frame (i.e. the NDPA frame may beomnidirectionally transmitted).

In this way, since both the NDP frame and the Beamforming Report Pollframe, transmitted by an STA (e.g., an AP) requesting feedback in thesounding procedure, have features in common in that they are frames fortriggering the feedback frame from target STAs (e.g., STA1, STA2, STA3,and STA4) of the sounding procedure, the NDP frame and the BeamformingReport Poll frame may be collectively referred to as feedback triggerframes.

In the example of FIG. 18, an AP corresponding to a beamformer transmitsthe NDPA frame to STA1, STA2, STA3, and STA4 corresponding tobeamformees and transmits the NDP frame (i.e. the PPDU frame onlyincluding a legacy preamble (i.e., an L-STF, an L-LTF, and L-SIG field)and a HE-preamble (i.e. a HE-SIG-A field, a HE-STF, and HE-LTF)) afteran SIFS. STA1, STA2, STA3, and STA4 corresponding to the beamformees maybe aware that they are target STAs of NDP sounding by receiving the NDPAframe and the NDP frame and perform channel estimation using the NDPframe. After receiving the feedback trigger frame (e.g., the NDP frameor the Beamforming Report Poll frame), each STA transmits the result(e.g., CSI) of channel estimation to the AP through a HE compressedbeamforming frame after an SIFS.

In the example of FIG. 18, a plurality of STAs simultaneously transmitsHE compressed beamforming frames (e.g., frames including the CSI).

Specifically, in the example of FIG. 18, it is assumed that beamformeeSpace Time Stream (STS) capabilities of STA1 and STA2 indicate that amaximum of two STSs is supported and beamformee STS capabilities of STA3and STA4 indicate that a maximum of four STSs is supported. In thiscase, the NDP frame cannot request channel estimation for STSs exceedingthe number of beamformee STS capabilities. For example, if beamformeeSTS capabilities indicate that a maximum of two STSs is supported, theNDP frame transmitted by the beamformer may request channel estimationfor the two STSs and cannot request channel estimation for more than twoSTSs (e.g., 3, 4, 5, 6, 7, or 8 STSs).

Since the number of HE-LTFs in the HE PPDU is determined based on thenumber of STSs as described above, the number of HE-LTFs used forchannel estimation for STSs in the NDP frame may be determined based onthe beamformee STS capabilities. That is, as in the example of FIG. 18,the NDP frame for STA1 and STA2 supporting a maximum of two STSs mayinclude two HE-LTFs and the NDP frame for STA3 and STA4 supporting amaximum of four STSs may include four HE-LTFs.

Furthermore, in NDP sounding according to the present invention,simultaneously performing NDP sounding with respect to STAs havingdifferent beamformee STS capabilities may be supported. As in theexample of FIG. 18, NDP sounding over a low channel (e.g., one 20 MHzchannel of a low frequency side out of two 20 MHz channels constitutinga 40 MHz channel when NDP sounding is performed over a 40 MHz channel inFIG. 18) for STA1 and STA2, beamformee STS capabilities of which supporttwo STSs, and NDP sounding over a high channel (e.g., one 20 MHz channelof a high frequency side out of two 20 MHz channels constituting a 40MHz channel) for STA3 and STA4, beamformee STS capabilities of whichsupport four STSs, may be simultaneously performed.

Specifically, in the example of FIG. 18, the AP may request that STAstransmit CSI on a channel (or subchannel) basis and receive the CSI. Forexample, the AP may perform NDP sounding using a 40 MHz channel withrespect to STA1, STA2, STA3, and STA4. In this case, the AP may transmitthe NDPA frame and the NDP frame over a low 20 MHz channel for thepurpose of requesting that STA1 and STA2 transmit the CSI for a low 20MHz channel and then the AP may receive the CSI of a low 20 MHz channelthrough the compressed beamforming frame from STA1 and STA2. At the sametime, the AP may transmit the NDPA frame and the NDP frame over a high20 MHz channel for the purpose of requesting that STA3 and STA4 transmitCSI for a high 20 MHz channel and then the AP may receive the CSI of ahigh 20 MHz channel through the compressed beamforming frame from STA3and STA4.

The NDPA frame transmitted over a low channel may include STA1 and STA2in an NDP sounding target STA list and the NDPA frame transmitted over ahigh channel may include STA3 and STA4 in the NDP sounding target STAlist. The NDPA frames over a low channel and a high channel may besimultaneously transmitted.

The NDP frame transmitted after an SIFS from an NDPA frame end timepoint over a low channel may include two HE-LTFs for channel estimationfor two STSs corresponding to beamformee STS capabilities of STA1 andSTA2. The NDP frame transmitted after an SIFS from an NDPA frame endtime point over a high channel may include four HE-LTFs for channelestimation for four STSs corresponding to beamformee STS capabilities ofSTA3 and STA4. The NDP frames over a low channel and a high channel maybe simultaneously transmitted.

If the number of HE-LTFs of the NDP frame transmitted over a low channelis different from that of the NDP transmitted over a high channel,transmission times of the NDP frames (i.e. NDP frame end time points)may be different. In this case, HE compressed beamforming framestransmitted as an immediate response to the NDP frames may becomedifferent over a low channel and a high channel.

To simultaneously transmit the HE compressed beamforming frames over alow channel and a high channel, the transmission times of the NDP frames(i.e., lengths of the NDP frames) over a plurality of channels may beset to be equal. To equally match the length of the NDP frametransmitted over a low channel with the length of the NDP frametransmitted over a high channel as in the example of FIG. 18, a paddingfield may be included in the NDP frame transmitted over a low channelafter a HE preamble. The padding field may correspond to additionaltransmission of a field of the HE preamble (e.g., a HE-LTF field or aHE-SIG-B field) or may be configured by a signal having a predeterminedpattern. If the padding field is added to any NDP frame, the length ofthe padding field may be set to a length difference between any NDPframe and another simultaneously transmitted NDP frame (e.g., differencebetween the length of the NDP frame transmitted over a high channel andthe length of the NDP frame transmitted over a low channel).

FIG. 19 depicts an exemplary NDPA frame format according to the presentinvention.

In the example of FIG. 19, a frame format of an MPDU included in a PSDUof a PPDU (or a HE PPDU) corresponding to an NDPA is illustrated.

A Receiver Address (RA) field may be set to an address of a receivingSTA of a corresponding frame. If an NDPA frame includes one STAInformation (Info) field, the RA field may be set to an address of acorresponding STA. If the NDPA frame includes a plurality of STA Infofields, the RA field may be set to a broadcast address.

A Transmitter Address (TA) field may be set to an address of atransmitting STA of a corresponding frame.

A Sounding Dialog Token field may be set to a value selected by abeamformer (e.g., an AP) in order to identify the NDPA frame.

The STA Info field may include information for designating beamformee(s)corresponding to a target STA of NDP sounding. The NDPA frame mayinclude one or multiple STA Info fields.

One STA Info field may include at least one of an Association Identifier(AID) subfield, a Feedback Type subfield, an Nc Index subfield, an NrIndex subfield, a Grouping subfield, and a Codebook Informationsubfield.

The AID subfield (e.g., 12-bit size) may include 12 Least SignificantBits (LSBs) of an AID of an STA that is expected to prepare soundingfeedback by processing an NDP frame following the NDPA frame. If the STAis an AP, a mesh STA, or a member of an IBSS, the AID subfield may beset to a value of 0.

The Feedback Type subfield (e.g., 1-bit size) may indicate a requestedfeedback type. If the Feedback Type subfield is set to 0, this mayindicate SU type feedback and, if the Feedback Type subfield is set to1, this may indicate MU type feedback.

The Nc Index subfield (e.g., 3-bit size) may be set to a value obtainedby subtracting one from the number of columns of a CompressedBeamforming Feedback Matrix subfield. If Nc=1, 2, 3, . . . , 8, then theNc Index subfield may be set to a value of 0, 1, 2, . . . , 7,respectively.

Herein, the Compressed Beamforming Feedback Matrix subfield may beincluded in a Compressed Beamforming Report field of a HE CompressedBeamforming frame and may include information about a compressedbeamforming feedback matrix to be applied to a specific subcarrier. Thecompressed beamforming feedback matrix may have a plurality of rows andone or more columns and elements of the matrix may indicate respectivespecific angles. The compressed beamforming feedback matrix fed backthrough the Compressed Beamforming Report field may be used for atransmission beamformer to determine a steering matrix.

The Nr Index subfield (e.g., 3-bit size) may be set to a value obtainedby subtracting one from the number of rows of the compressed beamformingfeedback matrix. If Nr=1, 2, 3, . . . , 8, then the Nr Index subfieldmay be set to a value of 0, 1, 2, . . . , 7, respectively.

The Grouping subfield (e.g., 2-bit size) may be set to a valueindicating a subcarrier grouping level, i.e., Ng, used for thecompressed beamforming feedback matrix. If Ng=1 (i.e., if there is nosubcarrier grouping), the Grouping subfield may be set to a value of 0.If Ng=2 (i.e., if only one compressed beamforming feedback matrix isreported with respect to a group including two adjacent subcarriers),the Grouping subfield may be set to a value of 1. If Ng=4 (i.e., if onlyone compressed beamforming feedback matrix is reported with respect to agroup including four adjacent subcarriers), the Grouping subfield may beset to a value of 2. The Grouping subfield set to a value of 3 may bereserved.

The Codebook Information subfield (e.g., 1-bit size) may be set to avalue indicating the size of a codebook entry.

Upon receiving the NDPA frame and the NDP frame following the NDPAframe, configured as described above, beamformees (e.g., STAs) maytransmit a plurality of HE compressed beamforming frames to a beamformer(e.g., AP) over channels receiving the NDPA and the NDP.

Herein, the multiple HE compressed beamforming frames transmitted by themultiple beamformees (e.g., multiple HE compressed beamforming frames asan immediate response to the NDP frame or multiple HE compressedbeamforming frames transmitted as an immediate response to theBeamforming Report Poll frame) may be simultaneously transmitted basedon configuration by the beamformer. That is, the multiple beamformeesmay simultaneously transmit the multiple HE compressed beamformingframes based on the NDPA frame transmitted from the beamformer to themultiple beamformees. For example, the multiple beamformees maysimultaneously transmit the multiple HE compressed beamforming framesbased on information (e.g., one or more of the Feedback Type, Nc Index,Nr Index, Grouping, and Codebook Information subfields of the STA Infofield) included in the NDPA frame transmitted from the beamformer to themultiple beamformees.

To simultaneously transmit the multiple HE compressed beamforming framesby the multiple beamformees, the sizes of the HE compressed beamformingframes transmitted by the multiple beamformees may be identically set.The sizes of the HE compressed beamforming frames may be determinedbased on the information included in the NDPA frame (e.g., one or moreof the Feedback Type, Nc Index, Nr Index, Grouping, and CodebookInformation subfields of the STA Info field). Specifically, each pieceof information included in the NDPA frame provided to the multiplebeamformees by the beamformer (e.g., one or more of the Feedback Type,Nc Index, Nr Index, Grouping, and Codebook Information subfields of theSTA Info field) may be set to the same value with respect to themultiple beamformees that simultaneously transmit the multiplecompressed beamforming frames. Referring back to FIG. 18, the sizes ofthe HE compressed beamforming frames of STA1 and STA2 may be equally setand the sizes of the HE compressed beamforming frames of STA3 and STA4may be equally set.

FIG. 20 depicts another exemplary sounding protocol according to thepresent invention.

FIG. 20 illustrates an additional example for simultaneously performingNDP sounding with respect to STAs having different beamformee STScapabilities. To this end, a plurality of durations corresponding todifferent beamformee STS capabilities may be consecutively transmittedin a transmission duration of a HE-LTE field used for channel estimationfor an STS in an NDP frame format.

In the example of FIG. 20, it may be assumed that beamformee STScapabilities of STA1 and STA2 are two STSs and beamformee STScapabilities of STA3 and STA4 are four STSs. In this case, the number ofHE-LTFs out of a HE preamble transmitted after a legacy preamble (anL-STF, an L-LTF, and an L-SIG field) in an NDP frame may be a total of6. That is, the HE-LTFs of the NDP frame may include HE-LTFs adding twoHE-LTFs for channel estimation of two STSs for STA1 and STA2 and fourHE-LTFs for channel estimation of four STSs for STA3 and STA4.

In this case, information indicating which HE-LTF is used for channelestimation of a certain STA (or an STS of which STA) may be included inthe NDPA frame.

As such, STA1 or STA2 may transmit a HE compressed beamforming frameincluding a channel estimation value (e.g., CSI) calculated based on thefirst two HE-LTFs in a HE preamble of the NDP frame to the AP as animmediate response to the NDP frame or an immediate response to aBeamforming Report Poll frame. STA3 or STA4 may transmit a HE compressedbeamforming frame including a channel estimation value (e.g., CSI)calculated based on the next four HE-LTFs of the first two HE-LTFs inthe HE preamble of the NDP frame to the AP as an immediate response tothe NDP frame or as an immediate response to the Beamforming Report Pollframe.

FIG. 21 depicts another exemplary sounding protocol according to thepresent invention.

FIG. 21 illustrates an additional example for simultaneously performingNDP sounding with respect to STAs having different beamformee STScapabilities. To this end, two different NDP frames may be transmittedat an interval of an SIFS after NDPA frame transmission.

In the example of FIG. 21, it may be assumed that beamformee STScapabilities of STA1 and STA2 are two STSs and beamformee STScapabilities of STA3 and STA4 are four STSs. In this case, two HE-LTFsfor channel estimation of two STSs for STA1 and STA2 may be included inthe first transmitted NDP frame and four HE-LTFs for channel estimationof four STSs for STA3 and STA4 may be included in the second transmittedNDP frame.

In this case, information indicating which NDP frame is used for channelestimation of a certain STA (or an STS of which STA) may be included inthe NDPA frame.

As such, STA1 or STA2 may transmit a HE compressed beamforming includinga channel estimation value (e.g., CSI) calculated based on two HE-LTFsof the first NDP frame to the AP as an immediate response to the lastNDP frame or an immediate response to the Beamforming Report Poll frame.STA3 or STA4 may transmit a HE compressed beamforming frame including achannel estimation value (e.g., CSI) calculated based on four HE-LTFs ofthe second NDP frame to the AP as an immediate response to the NDP frameor as an immediate response to the Beamforming Report Poll frame.

Although the NDPA frame is transmitted to STA1 and STA3, and STA1 andSTA3 transmit the HE compressed beamforming frames in the illustratedexamples of FIGS. 20 and 21, the present invention is not limitedthereto. That is, the NDPA frame for designating one or more of STA1,STA2, STA3, and STA4 as NDP sounding target STAs may be transmitted fromthe AP, one NDP frame (i.e. the example of FIG. 20) or multiple NDPframes (i.e. the example of FIG. 21) for channel estimation for thedesignated NDP sounding target STAs may be transmitted from the AP, andthe HE compressed beamforming frame from each of the designated targetSTAs may be transmitted to the AP as an immediate response to the NDPframe or as an immediate response to the Beamforming Report Poll frame.

The above-described examples of the present invention represent that HEcompressed beamforming frames are sequentially transmitted from multipleSTAs over one channel (e.g., a 20 MHz channel). That is, as illustratedin FIG. 22, a HE compressed beamforming frame from one STA may betransmitted as a response to the NDPA frame (or NDPA and NDP frames) oras a response to the Beamforming Report Poll frame over one channel anda HE compressed beamforming frame from another STA may be transmitted asa response to an additional Beamforming Report Poll frame.

FIG. 23 depicts another exemplary sounding protocol according to thepresent invention.

In the illustrated example of FIG. 23, multiple STAs simultaneouslyreport HE compressed beamforming feedback over one channel in a WLANsystem supporting UL MU transmission (e.g., UL MU-MIMO transmission orUL OFDMA transmission). That is, a HE PPDU including HE compressedbeamforming feedback from multiple STAs over one channel may betransmitted by a UL MU transmission mode.

In this way, simultaneous transmission of the HE compressed beamformingframes (i.e., by the UL MU transmission mode) by multiple STAs may beset or indicated through an NDPA frame or a Beamforming Report Pollframe of a sounding protocol having the multiple STAs as target STAs.

As illustrated in FIG. 23, the HE NDPA frame or the Beamforming ReportPoll frame may further include information initiating or eliciting UL MUtransmission (e.g., UL MU transmission trigger information or UL MUtransmission scheduling information). Alternatively, the HE NDPA frameor the Beamforming Report Poll frame may include a trigger frameinitiating or eliciting UL MU transmission. For example, the HE NDPAframe and the trigger frame eliciting UL MU transmission may beintegrated and the Beamforming Report Poll frame and the trigger frameeliciting UL MU transmission may be integrated.

HE beamformees (e.g., STA1, STA2, STA3, and STA4) may receive the HENDPA frame including UL MU transmission trigger information from a HEbeamformer (e.g., AP) with which the HE beamformees are associated andreceive a HE NDP frame after an SIFS from reception of the HE NDPAframe. In this case, the HE beamformees may simultaneously transmit HEPPDUs including HE compressed beamforming feedback thereof by a UL OFDMAmode or a UL MU-MIMO mode after an SIFS from reception of the HE NDPframe.

In addition, upon receiving a Beamforming Report Poll frame includingthe UL MU transmission trigger information from the HE beamformer (e.g.,AP) with which the HE beamformees (e.g., STA1, STA2, STA3, and STA4) areassociated, the HE beamformees may simultaneously transmit the HE PPDUsincluding the HE compressed beamforming feedback thereof by the UL OFDMmode or the UL MU-MIMO mode after an SIFS from reception of theBeamforming Report Poll frame.

Even when the sounding protocol is performed over each of a plurality ofchannels (e.g., similarly to the example of FIG. 18, a low 20 MHzchannel and a high 20 MHz channel), the trigger information for UL MUtransmission may be included in the NDPA frame and a plurality of STAsthat received the NDP frame after the NDPA frame may simultaneouslytransmit the HE compressed beamforming frames using the UL MUtransmission mode. Further, the trigger information for UL MUtransmission may be included in the Beamforming Report Poll frametransmitted from the AP over each channel and a plurality of STAs thathave received the Beamforming Report Poll frame may simultaneouslytransmit the HE compressed beamforming frames by the UL MU transmissionmode. For example, STA1 and STA2 that have received the NDPA frame (orNDPA frame and NDP frame) or the Beamforming Report Poll frame over alow 20 MHz channel may simultaneously transmit the HE compressedbeamforming frames by the UL MU transmission mode and STA3 and STA4 thathave received the NDPA frame (or NDPA frame and NDP frame) or theBeamforming Report Poll frame over a high 20 MHz channel maysimultaneously transmit the HE compressed beamforming frames using theUL MU transmission mode.

Herein, to identically set transmission times of the HE compressedbeamforming feedback transmitted from the multiple beamformees, groupinglevels of the HE compressed beamforming feedback, i.e., Ng, may beequally set with respect to the multiple beamformees. The groupinglevels set with respect to the multiple beamformees may be provided tothe multiple beamformees through the NDPA frame.

In this way, the multiple beamformees may simultaneously transmit the HEcompressed beamforming frames by the UL MU transmission mode by settingor indication through the NDPA frame or the Beamforming Report Pollframe in the sounding protocol for the multiple beamformees.

The beamformer may determine whether to apply beamforming to each of themultiple beamformees in consideration of the HE compressed beamformingfeedback (e.g., CSI) reported from the multiple beamformees and transmitthe HE PPDU to which beamforming is applied according to the determinedresult.

FIGS. 24 and 25 depict exemplary DL HE PPDU frame formats supporting DLbeamforming transmission according to the present invention.

If an AP transmits a beamformed DL HE PPDU to a plurality of STAs, aparameter indicating whether beamforming is applied to each of themultiple STA (or each of multiple subchannels) (e.g., a “Beamformed”parameter) may be encoded and then included in a HE-SIG field (e.g.,HE-SIG-A field or HE-SIG-B field) of the DL HE PPDU.

If the Beamformed parameter for any STA is set to True (i.e., if theparameter indicates that beamforming is applied to a corresponding STAor a subchannel allocated to a corresponding STA), smoothing may not beapplied.

In the example of FIG. 24, the AP may transmit a DL OFDMA HE PPDUincluding the Beamformed parameter for each of STA1, STA2, STA3, andSTA4 to STA1, STA2, STA3, and STA4. Herein, it is assumed that theBeamformed parameter for each of STA1 and STA4 is set to True.Therefore, a beamforming steering matrix may be applied to a HE-STF, aHE-LTF, and a PSDU having STA1 as a target STA and the beamformingsteering matrix may be applied to the HE-STF, the HE-LTF, and the PSDUhaving STA4 as the target STA. In this case, smoothing may not beapplied to STA1 and STA4 receiving a beamformed DL HE PPDU. Meanwhile,the beamforming steering matrix may not be applied to the HE-STF, theHE-LTF, and the PSDU having STA2 and STA3 as target STAs.

Thus, the Beamformed parameter included in the HE-SIG field of the HEPPDU may be user-specifically (or STA-specifically orbeamformee-specifically) set.

As an additional example of the present invention, the Beamformedparameter included in the HE-SIG field of the HE PPDU may also besubchannel-specifically set.

In FIG. 25, an AP may transmit a DL OFDMA HE PPDU including a Beamformedparameter for each of subchannel1 (CH1), subchannel2 (CH2), subchannel3(CH3), and subchannel4 (CH4) to STA1, STA2, STA3, STA4, STA5, STA6,STA7, and STA8. Herein, it is assumed that the Beamformed parameter foreach of CH1 and CH4 is set to True. Then, a beamforming steering matrixmay be applied to a HE-STF, a HE-LTF, and a PSDU (i.e., having STA1 andSTA5 as target STAs) transmitted over CH1 and the beamforming steeringmatrix may be applied to the HE-STF, the HE-LTF, and the PSDU (i.e.,having STA4 and STA8 as target STAs) transmitted over CH4. In this case,smoothing may not be applied to STA1, STA4, STA5, and STA8 receiving abeamformed-applied DL HE PPDU over CH1 and CH4. On the other hand, thebeamforming steering matrix may not be applied to the HE-STF, theHE-LTF, and the PSDU (having STA2 and STA3 as target STAs and havingSTA6 and STA7 as target STAs) transmitted over each of CH2 and CH3.

Meanwhile, since the beamforming steering matrix may always be appliedto a transmission signal waveform with respect to the DL HE PPDUtransmitted by a DL MU-MIMO mode, the Beamformed parameter may not beincluded in the HE PPDU.

FIGS. 26 to 30 depict exemplary trigger frame formats and UL HE PPDUframe formats supporting UL beamforming according to the presentinvention.

If an AP transmits a trigger frame initiating or eliciting transmissionof a beamformed UL HE PPDU to a plurality of STAs, the STAs may transmita UL HE PPDU frame to the AP as an immediate response to the triggerframe (e.g., after an SIFS from reception of the trigger frame).

When the beamformed UL HE PPDU is transmitted from the multiple STAs, aparameter indicating whether beamforming is applied to each of themultiple STA (or each of multiple subchannels) (e.g., the “Beamformed”parameter) may be encoded and then included in a HE-SIG field (e.g.,HE-SIG-A field or HE-SIG-B field) of the UL HE PPDU.

As in the example of FIG. 26, when STA1, STA2, STA3, and STA4 transmit abeamformed or non-beamformed UL HE PPDU, the UL HE PPDU may include theBeamformed parameter for each of STA1, STA2, STA3, and STA4 in theHE-SIG field (e.g., HE-SIG-A field or HE-SIG-B field). In the example ofFIG. 26, it is assumed that the Beamformed parameter for each of STA1and STA4 is set to True.

If the HE-SIG field (e.g., HE-SIG-B field) of the UL HE PPDU includescontrol information commonly applied to the multiple STAs (or multiplesubchannels), the Beamformed parameter may not be included in the UL HEPPDU. Since the AP determines the Beamformed parameter of the UL HE PPDUand informs all STAs, that are to transmit the UL HE PPDU, of theBeamformed parameter, it is not necessary for the STAs to inform the APof the Beamformed parameter in transmission of the UL HE PPDU.Therefore, the Beamformed parameter may be included in the UL HE PPDUonly when the control information of the HE-SIG field of the UL HE PPDUis not commonly applied to the multiple STAs (or multiple subchannels).

As an additional example of the present invention, the Beamformedparameter for each STA (or each subchannel) may not be included in theHE-SIG field (e.g., the HE-SIG-A or HE-SIG-B field) of the UL HE PPDU.Further, the UL HE PPDU frame may not include the HE-SIG-B field. Inthis case, the trigger frame eliciting UL MU transmission may include asmoothing parameter indicating whether to apply smoothing to each of themultiple STAs (or each of the multiple subchannels) with respect to theUL HE PPDU received by the AP.

When the smoothing parameter included in the trigger frame is set toTrue for any STA (or any subchannel), beamforming may not be applied totransmission from a corresponding STA (or a corresponding subchannel) inthe UL HE PPDU. That is, since it is desirable that smoothing not beapplied to an AP receiving beamformed UL transmission, an STA may notapply beamforming to UL transmission to which the AP has indicated thatsmoothing will be applied. This may be understood as meaning that the APindirectly or implicitly indicates that beamforming is notuser-specifically or subchannel-specifically applied to UL transmission.

If the smoothing parameter included in the trigger frame is set to Falsewith respect to any STA (or any subchannel), beamforming may or may notbe applied to transmission from the corresponding STA (or over thecorresponding subchannel) in the UL HE PPDU. That is, since the APreceiving beamformed UL transmission does not desirably apply smoothingto the beamformed UL transmission and may apply or may not applysmoothing receiving non-beamformed UL transmission, the STA may apply ormay not apply beamforming to UL transmission. Therefore, the STA mayapply or may not apply beamforming to UL transmission to which the APindicates that smoothing will not be applied. This may be understood asmeaning that whether beamforming is applied to UL transmission to whichthe AP indicates that smoothing will not be applied is determined by theSTA.

In the example of FIG. 27, the HE-SIG-B field may not be included (ormay be omitted) in the UL HE PPDU and whether beamforming is applied toeach STA (or each subchannel) may be determined based on a smoothingoperation of a beamformer (i.e. AP) indicated by the trigger frame. Ifthe smoothing parameter included in the trigger frame is set to Truewith respect to each of STA2 and STA3, a beamforming steering matrix isnot applied to a PSDU (or a HE-STF, a HE-LTF, and a PSDU or a legacypreamble, a HE-SIG-A, a HE-STF, a HE-LTF, and a PSDU) transmitted byeach of STA2 and STA3. Meanwhile, if the smoothing parameter included inthe trigger frame is set to False with respect to each of STA1 and STA4,the beamforming steering matrix may or may not be applied to a PSDU (ora HE-STF, a HE-LTF, and a PSDU or a legacy preamble, a HE-SIG-A, aHE-STF, a HE-LTF, and a PSDU) transmitted by each of STA1 and STA4 andthis may be determined by each of STA1 and STA4.

As an additional example of the present invention, the Beamformedparameter may be included in the trigger frame instead of the smoothingparameter. That is, if the Beamformed parameter for each STA (or eachsubchannel) is not included in the HE-SIG field (e.g., the HE-SIG-A orHE-SIG-B field) of the UL HE PPDU, the Beamformed parameter indicatingwhether beamforming is applied to each of multiple STAs (or each ofmultiple subchannels) with respect to the UL HE PPDU received by the APmay be included in the trigger frame eliciting UL MU transmission.Unlike the smoothing parameter indirectly or implicitly indicatingwhether beamforming is applied in the UL HE PPDU, the Beamformedparameter may directly or explicitly indicate whether beamforming isapplied in the UL HE PPDU.

If the Beamformed parameter included in the trigger frame is set to Truewith respect to any STA (or any subchannel), beamforming may be appliedto transmission from the corresponding STA (or over the correspondingsubchannel) in the UL HE PPDU.

If the Beamformed parameter included in the trigger frame is set toFalse with respect to any STA (or any subchannel), beamforming may notbe applied to transmission to the corresponding STA (over thecorresponding subchannel) in the UL HE PPDU.

In the example of FIG. 28, the HE-SIG-B field may not be included (ormay be omitted) in the UL HE PPDU and whether beamforming is applied toeach STA (or each subchannel) may be determined based on the Beamformedparameter indicated by the trigger frame. If the Beamformed parameterincluded in the trigger frame is set to False with respect to each ofSTA2 and STA3, the beamforming steering matrix is not applied to a PSDU(or a HE-STF, a HE-LTF, and a PSDU or a legacy preamble, a HE-SIG-A, aHE-STF, a HE-LTF, and a PSDU) transmitted by each of STA2 and STA3.Meanwhile, if the Beamformed parameter included in the trigger frame isset to True with respect to each of STA1 and STA4, the beamformingsteering matrix is applied to a PSDU (or a HE-STF, a HE-LTF, and a PSDUor a legacy preamble, a HE-SIG-A, a HE-STF, a HE-LTF, and a PSDU)transmitted by each of STA1 and STA4.

FIGS. 29 and 30 depict exemplary parts to which beamforming is appliedin a UL HE PPDU according to the present invention.

The example of FIG. 29 indicates the case in which durations duringwhich applying beamforming is permitted in a UL HE PPDU are a HE-STF, aHE-LTF, and a PSDU and the example of FIG. 30 indicates the case inwhich durations during which applying beamforming is permitted are alldurations of the UL HE PPDU (i.e., a legacy preamble, a HE-SIG-A, aHE-STF, a HE-LTF, and a PSDU).

Specifically, in the example of FIG. 29, a beamforming steering matrixmay not be applied to a legacy preamble (an L-STF, an L-LTF, and anL-SIG field) and a HE-SIG-A field in the UL HE PPDU and the beamformingsteering matrix may or may not be applied to a HE-STF, a HE-LTF, and aPSDU according to indication of an AP or according to determination byan STA. In this case, even a legacy STA that does not recognize a HEPPDU frame format may decode LENGTH information of a legacy preamble(e.g., L-SIG) of a HE PPDU and determine that a channel is occupied fora corresponding time, thereby protecting transmission of the HE PPDU.Meanwhile, the legacy preamble and the HE-SIG-A field have lowbeamforming gain because beamforming is not applied thereto.

In the example of FIG. 30, a beamforming steering matrix may or may notbe applied to the legacy preamble (an L-STF, an L-LTF, and an L-SIGfield), a HE-SIG-A field, a HE-STF, a HE-LTF, and a PSDU according toindication of an AP or according to determination by an STA. In thiscase, although beamforming gain for the legacy preamble and the HE-SIG-Afield may be obtained, a legacy STA cannot decode the legacy preamble ofthe HE PPDU.

Meanwhile, as illustrated in FIGS. 24 to 26, the beamforming steeringmatrix may not be applied to a part (e.g., the legacy preamble, theHE-SIG-A field, and the HE-SIG-B field) in the DL HE PPDU and thebeamforming steering matrix may or may not be applied to the other parts(e.g., the HE-STF, the HE-LTF, and the PSDU) in the DL HE PPDU.Alternatively, the beamforming steering matrix may or may not be appliedto all durations (e.g., the legacy preamble, the HE-SIG-A field, theHE-SIG-B field, the HE-STF, the HE-LTF, and the PSDU) in the DL HE PPDU.

FIG. 31 is a flowchart depicting an exemplary method according to thepresent invention.

In step S3110, an AP may transmit an NDPA frame to a plurality STAs toinform the STAs that the AP will transmit an NDP frame after the NDPAframe. The NDPA frame may include a list indicating the plurality ofSTAs. The NDPA frame may further include trigger information for UL MUtransmission from N (N≥2) STAs among the plurality of STAs.

In step S3120, the AP may transmit the NDP frame following the NDPAframe to the STAs.

If the trigger information for UL MU transmission is not included in theNDPA frame transmitted in step S3110, the first STA of the STA listincluded in the NDPA frame may receive the NDP frame even withoutreceiving an additional polling frame (e.g., a Beamforming Report Pollframe) from the AP and transmit a beamforming feedback report to the APin step 3130 immediately after receiving the NDP frame (e.g., after anSIFS from an NDP frame end time).

If the trigger information for UL MU transmission from N (N≥2) STAs isincluded in the NDPA frame transmitted in step S3110, the AP maysimultaneously receive beamforming feedback reports (e.g., HE compressedbeamforming feedback frames) from the N STAs in step S3130. The feedbackreports from the N STAs may include beamforming associated information(refer to the example of FIG. 19) included in the NDPA frame and achannel estimation result (e.g., CSI) based on a HE-LTF, etc. of the NDPframe. In addition, simultaneous transmission of the beamformingfeedback reports from the N STAs (e.g., HE compressed beamforming framesof a UL MU transmission mode) may be performed according to the triggerinformation for UL MU transmission included in the NDPA frame.

If there are STAs that have not transmitted the beamforming feedbackreports in step S3130 among the STAs of the STA list of the NDPA frame,steps S3140 and S3141, which will be described below, may be performedwith respect to some or all of the STAs that have not transmitted thebeamforming feedback reports.

In step S3140, the AP may transmit the Beamforming Report Poll frame toa group including one or more additional STAs and the one or more STAsmay transmit the beamforming feedback reports (e.g., HE compressedbeamforming feedback frames) to the AP in response to the BeamformingReport Poll frame. The Beamforming Report Poll frame may further includethe trigger information for UL MU transmission.

If the Beamforming Report Poll frame is transmitted to one STA and thetrigger information for UL MU transmission is not included in theBeamforming Report Poll frame in step S3140, the one STA receiving theBeamforming Report Poll frame may transmit the beamforming feedbackreport to the AP in step S3141.

If the Beamforming Report Poll frame is transmitted to two or more STAs(i.e., M≥2) in step S3140 and the Beamforming Report Poll frame includesthe trigger information for UL MU transmission from the two or moreSTAs, the two or more STAs receiving the Beamforming Report Poll framemay simultaneously transmit the beamforming feedback reports (e.g., HEcompressed beamforming feedback frames) to the AP in step S3141.

If there are STAs that have not transmitted the beamforming feedbackreports in steps S3130 and S3141 among the STAs of the STA list of theNDPA frame, steps S3150 and S3151 may be performed with respect to theSTAs that have not transmitted the beamforming feedback reports. StepsS3150 and S3151 correspond to steps S3140 and S3141 and representtransmission of the Beamforming Report Poll frame for a group includingone or more additional STAs and transmission of the beamforming feedbackreports in response to the Beamforming Report Poll frame.

In this way, the beamforming feedback reports may be transmitted to theAP from all of the STAs of the STA list included in the NDPA framethrough step S3130 (or steps S3130 to S3151).

In step S3160, the AP may determine whether to apply beamforming to aplurality of data units (e.g., PSDUs) transmitted to the STAs based onthe beamforming feedback reports from the STAs.

In step S3170, the AP determines beamforming steering matrices to beapplied to transmission to corresponding STAs based on the beamformingfeedback report from the corresponding STAs with respect to one or moreSTAs (i.e., beamformee STAs) to which beamforming is determined to beapplied.

In step S3180, the AP may transmit a DL HE PPDU to the multiple STAs.The DL HE PPDU may include beamforming information indicating whetherbeamforming is individually applied to each of the multiple data units(or multiple subchannels or multiple STAs) in a HE-SIG field (e.g., aHE-SIG-B field). In addition, the AP may transmit the DL HE PPDU byindividually applying beamforming (i.e., by applying the beamformingsteering matrix determined for a corresponding STA) or by not applyingbeamforming to each of the data units (or subchannels or STAs) accordingto the beamforming information.

FIG. 32 is a flowchart depicting another exemplary method according tothe present invention.

An AP may transmit a trigger frame to an STA group (i.e., a groupincluding one or more STAs) in step S3210. An STA receiving the triggerframe may simultaneously generate a UL HE PPDU together with one or moreother STAs (i.e. using a UL MU transmission mode). The trigger frame mayinclude scheduling information for UL MU transmission and may includebeamforming information indicating whether to individually applybeamforming to each of a plurality of data units (or a plurality ofsubchannels or STAs) of the UL HE PPDU.

In step S3220, each of the STAs belonging to the STA group may determinewhether to apply beamforming to a data unit (or a subchannel over whichthe data unit is transmitted) that each STA is to transmit based on thebeamforming information included in the trigger frame.

The STA that has determined that beamforming is applied for transmissionof the UL data unit in step S3220 may determine a beamforming steeringmatrix to be applied to the data unit that the STA transmits in stepS3230. The STA may determine the beamforming steering matrix based oninformation included in the trigger frame. In step S3240, the STA maysimultaneously transmit a beamformed data unit (i.e., a data unit towhich the beamforming steering matrix is applied) to the AP togetherwith one or more other STAs (i.e., using the UL MU transmission mode).

The STA that has determined that beamforming is not applied fortransmission of the UL data unit in step S3220 may simultaneouslytransmit a non-beamformed data unit to the AP together with one or moreother STAs (i.e., using the UL MU transmission mode) in step S3250.

That is, any STA belonging to the STA group may transmit the UL HE PPDUincluding a beamformed or non-beamformed data unit according to thebeamforming information included in the trigger frame. In transmissionof the UL HE PPDU, one or more other beamformed or non-beamformed dataunits (or one or more other STAs or one or more other subchannels) maybe simultaneously transmitted to the AP together with the data unit ofthe STA.

While the exemplary method has been described with reference to FIGS. 31and 32 as a series of operations for simplicity of description, thisdoes not limit the sequence of steps. When needed, steps may beperformed at the same time or in a different sequence. All of theexemplary steps are not always necessary to implement the methodproposed by the present invention.

The foregoing embodiments of the present invention may be implementedseparately or combinations of two or more of the embodiments may beimplemented simultaneously, for the method of FIGS. 31 and 32.

The present invention includes an apparatus for processing or performingthe method of the present invention (e.g., the wireless device and itscomponents described with reference to FIGS. 1, 2, and 3).

The present invention includes software (an operating system (OS), anapplication, firmware, a program, etc.) for executing the method of thepresent invention in a device or a computer, and a medium storing thesoftware that can be executed in a device or a computer.

While various embodiments of the present invention have been describedin the context of an IEEE 802.11 system, they are applicable to variousmobile communication systems.

What is claimed is:
 1. A method for transmitting by an access point (AP)to a plurality of stations (STAs) a Physical layer Protocol Data Unit(PPDU) frame in a wireless local area network, the method comprising:transmitting a SIGNAL field of the PPDU frame, the SIGNAL fieldincluding beamforming information indicating whether beamforming isapplied to individual data units of the PPDU frame; and transmitting thedata units of the PPDU frame, the data units being individuallybeamformed or not beamformed according to the beamforming information,wherein the SIGNAL field has a same size as a number of subchannels in atransmission channel to the plurality of STAs.
 2. The method accordingto claim 1, wherein: the PPDU frame is transmitted on a transmissionchannel divided into a plurality of subchannels, the data units arerespectively transmitted on the plurality of subchannels, and thebeamforming information indicates whether the beamforming is applied torespective subchannels.
 3. The method according to claim 1, wherein: thedata units are respectively transmitted to the plurality of STAs, andthe beamforming information indicates whether the beamforming is appliedto respective STAs.
 4. The method according to claim 1, wherein theSIGNAL field is transmitted before a High Efficiency-Long Training Field(HE-LTF) of the PPDU frame.
 5. The method according to claim 1, whereinthe SIGNAL field is a High Efficiency-SIGNAL-B (HE-SIG-B) field.
 6. Themethod according to claim 1, wherein High-Efficiency Short TrainingFields (HE-STFs), HE-LTFs and the data units of the PPDU frame areindividually beamformed or not beamformed according to the beamforminginformation.
 7. The method according to claim 1, wherein whether thebeamforming is applied to individual data units is determined by the APbased on beamforming feedback reports simultaneously received from theplurality of STAs.
 8. The method according to claim 7, wherein thebeamforming feedback reports simultaneously received from the pluralityof STAs are elicited by trigger information provided from the AP to theplurality of STAs.
 9. The method according to claim 8, wherein thetrigger information is included in a Non-Data Packet Announcement (NDPA)frame or a Beamforming Report Poll frame.
 10. The method according toclaim 9, wherein: the NDPA frame includes at least one STA Informationfield, and each of the at least one STA Information field includes atleast one of Association Identifier (AID) subfield, Feedback Typesubfield, Nc Index subfield, Nr Index subfield, Grouping subfield, orCodebook Information subfield.
 11. The method according to claim 10,wherein: the Nc Index subfield is set to a number of columns ofCompressed Beamforming Feedback Matrix minus 1, and wherein the Nr Indexsubfield is set to a number of rows of Compressed Beamforming FeedbackMatrix minus
 1. 12. The method according to claim 9, wherein: at leastone NDP frame following the NDPA frame includes HE-LTFs, and a number ofthe HE-LTFs included in the at least one NDP frame is determined basedon total number of streams transmitted to the plurality of STAs.
 13. Amethod for transmitting by a station (STA) to an access point (AP) aPhysical layer Protocol Data Unit (PPDU) frame in a wireless local areanetwork, the method comprising: receiving a trigger frame eliciting thePPDU frame simultaneously transmitted from a plurality of STAs includingthe STA and at least one other STA, wherein a SIGNAL field of thetrigger frame includes beamforming information indicating whetherbeamforming is applied to individual data units of the PPDU frame; andtransmitting the PPDU frame including a data unit of the STA, the dataunit of the STA being individually beamformed or not beamformedaccording to the beamforming information, wherein the SIGNAL field has asame size as a number of subchannels in a transmission channel to theplurality of STAs.
 14. The method according to claim 13, wherein: thePPDU frame is transmitted on a transmission channel divided into aplurality of subchannels, the data units are respectively transmitted onthe plurality of subchannels, and the beamforming information indicateswhether the beamforming is applied to respective subchannels.
 15. Themethod according to claim 13, wherein: the data units are respectivelytransmitted from the plurality of STAs, and the beamforming informationindicates whether the beamforming is applied to respective STAs.
 16. Themethod according to claim 13, wherein: the data units are respectivelytransmitted from the plurality of STAs, and the beamforming informationindicates whether a smoothing is applied to respective STAs.
 17. Themethod according to claim 13, wherein a High-Efficiency Short TrainingField (HE-STF), a High Efficiency-Long Training Field (HE-LTF) and thedata unit of the STA in the PPDU frame are individually beamformed ornot beamformed according to the beamforming information.
 18. The methodaccording to claim 13, wherein a Legacy-STF (L-STF), a Legacy-LTF(L-LTF), a Legacy-SIGNAL (L-SIG), a HE-STF, a HE-LTF and the data unitof the STA in the PPDU frame are individually beamformed or notbeamformed according to the beamforming information.
 19. The methodaccording to claim 13, wherein the PPDU frame lacks HighEfficiency-SIGNAL-B (HE-SIG-B) field.